E.cpp

/**
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣀⣤⡤⠤⣤⣄⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡴⠛⠉⠀⠀⠀⠀⠈⠙⢷⣄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠟⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⣆⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣰⠶⣄⡀⠀⠀⠀⢠⡟⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢹⡆⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢰⡏⠀⠀⠉⠛⠶⣤⣸⡇⠀⠀⠀⠀⣀⣤⣶⣶⠒⠒⠒⠶⣬⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣀⣤⠴⠶⣿⠀⠀⠀⠀⠀⠀⠈⠉⠉⠛⠒⠶⢿⣭⣀⡀⢻⡀⠀⠀⢠⡿⠃⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣤⠶⠛⠋⠁⠀⠀⢸⠃⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠉⠛⠷⣴⣞⠛⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⡴⠞⠉⠀⠀⠀⠀⠀⠀⢰⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠛⢦⣄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⠞⠋⠀⠀⠀⠀⢀⡤⠠⡄⠀⢰⡏⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⠿⣄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⡞⠁⠀⠀⠀⠀⣠⠖⠋⠀⣸⠇⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⠀⢀⡀⠀⠀⠀⠀⠀⠀⢦⡀⠀⠀⠀⠸⣷⣄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡼⠋⠀⠀⠀⠀⢀⣴⠋⢀⢀⡴⠋⠀⢀⣼⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣽⠀⣼⢿⡄⠀⠀⠀⠀⣆⠀⠉⢦⡀⠀⠀⠀⠘⢧⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⡟⠁⠀⠀⠀⠀⣠⠟⠇⠀⠈⠉⠁⠀⣀⣾⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡟⢰⠏⠀⠻⣄⠀⠀⠀⠹⣄⣰⠟⠁⠀⠀⠀⠀⠀⢻⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⡟⠀⢀⣀⡖⠀⣰⠏⠀⠀⠀⠀⠀⢀⣼⣿⠋⠸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⣧⡟⠀⠀⠀⠹⣦⠀⠀⠀⠀⠁⠀⣶⠀⠀⣴⠛⢧⠀⢻⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⣠⠞⣩⡟⠀⠀⠈⠉⠀⢀⡟⠀⠀⢀⣀⣠⣤⣾⡿⠗⠒⠚⣿⠠⣤⠀⠀⠀⠀⠀⠀⠀⢸⣿⠓⠒⠲⠶⠶⠾⢷⣤⣀⣀⠀⠀⠙⣧⠀⠹⣆⣼⠃⠀⢷⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⢀⡴⠟⢁⡼⣿⠁⠀⠀⠀⠀⠀⢸⠃⠀⠀⠈⠉⢠⡾⠋⠀⠀⠀⠀⠸⣆⠙⣧⡀⠀⠀⠀⠀⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠻⣆⠉⠁⠀⠀⢹⡄⠀⠈⠁⠀⠀⠘⣷⢦⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⣠⡶⠋⢀⡴⠋⢰⡏⠀⠀⠀⠀⠀⠀⢸⠀⠀⠀⠀⣠⠟⠀⠀⠀⠀⠀⠀⠀⢻⡄⢸⣷⣄⠀⠀⠀⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠈⢳⣄⠀⠀⠸⣧⠀⠀⠀⠀⠀⠀⢿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⢰⣾⡥⠴⠞⠋⠀⠀⣼⠀⠀⠀⠀⠀⠀⠀⣸⠀⠀⠀⣰⠏⠀⠀⠀⠀⠀⠀⠀⠀⠀⢻⡄⣷⠙⠷⣄⡀⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⢷⣄⠀⣿⠀⠀⠀⠀⠀⠀⢸⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⣿⡆⠀⠀⠀⠀⠀⠀⢸⡄⠀⣼⢏⣀⣤⣶⣦⣤⣶⣶⣶⣶⣶⣿⣿⣾⡆⠀⠈⠻⢦⣼⡇⢰⣶⣶⣶⣶⣶⣶⣶⣤⣤⣤⣦⠙⢧⣿⠀⠀⠀⠀⠀⠀⢸⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⣿⡇⠀⠀⠀⠀⠀⠀⠘⣇⣼⠏⠘⣿⡿⢿⣿⣿⣿⣿⣿⡏⠉⠉⠉⠙⠃⠀⠀⠀⠀⠉⠁⠘⠛⢻⣿⣿⣿⣿⣿⣟⠛⢛⠷⠀⠀⣿⠀⠀⠀⠀⠀⠀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⡟⣷⠀⠀⠀⠀⠀⠀⠀⢻⡏⠀⠀⠀⠀⣸⣿⣿⣿⣿⣿⣷⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣾⣿⣿⣿⣿⣿⡇⠀⠀⠀⠀⢰⡏⠀⠀⠀⠀⠀⠀⣾⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⣷⢹⣆⠀⠀⠀⠀⠀⠀⠈⣷⠀⠀⠀⠀⢹⣿⣿⣿⣿⣿⣿⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣿⣿⣿⣿⣿⣿⡇⠀⠀⠀⠀⣾⠀⠀⠀⠀⠀⠀⢰⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⢿⠘⣿⣆⠀⠀⠀⠀⠀⠐⠘⣧⠀⠀⠀⠘⢿⣿⣿⣿⣿⠏⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⢿⣿⣿⣿⡿⠃⠀⠀⠀⣼⠃⠀⠀⠀⠀⠀⣰⠟⠐⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠸⣇⣿⠙⣧⣄⠀⠀⠀⠀⠀⠘⢧⡀⠀⠀⠈⢹⠿⢟⡋⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⢻⡿⠀⡀⠀⠀⣼⠃⠀⠀⠀⠀⣀⡾⠋⠀⣴⣷⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⢻⣿⠀⣿⠝⠳⣤⣀⡀⠀⠀⠈⢷⣤⠇⢠⡞⠠⠾⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢠⠟⠁⡾⠃⣠⡾⠃⠀⠀⣀⣤⠾⠋⠀⠀⠀⡿⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⣿⣸⣿⠀⠀⠀⠉⠙⠓⡶⠦⠤⣿⡆⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠓⢰⡿⠤⠴⠶⣿⠉⠀⠀⠀⠀⠀⢠⡇⢸⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⣿⢻⠀⠀⠀⠀⠀⠃⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢲⣄⠀⠀⠀⠀⠀⠀⠀⢀⣴⠆⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡟⠀⠀⠀⠀⠀⠀⢸⡇⣸⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢠⡿⢸⡇⠀⠀⠀⠀⠀⢿⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠛⠲⠤⣤⠤⠴⠞⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢰⡏⠀⠀⠀⠀⠀⠀⢸⠇⠈⣇⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣸⡇⢸⡇⠀⠀⠀⠀⠀⢸⣿⣦⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢿⡇⠀⠀⠀⠀⠀⠀⢺⢀⠀⢿⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣸⢷⠛⣇⠀⠀⠀⠀⠀⠈⣿⠉⠻⣦⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⣅⣸⠀⠀⠀⠀⠀⠀⢀⣿⢸⡀⢸⡇⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡿⠘⣀⣿⠀⠀⠀⠀⠀⢷⢸⡄⠀⠈⠙⠶⣤⣀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⡴⠟⠁⠀⠈⣿⠀⠀⠀⠀⠀⠀⠈⡟⣼⡇⠀⣧⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⢰⡇⠀⢿⢿⠀⠀⠀⠀⠀⠀⠈⣧⠀⠀⠀⠀⠀⠉⠛⠶⢤⣤⣀⣀⠀⣀⡀⠀⠀⠀⠀⠀⢀⣠⡴⠞⠋⠁⠀⠀⠀⠀⢀⡏⠀⠀⠀⠀⠀⠀⢠⡇⢹⣤⠀⢹⡄⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⣼⠀⠀⡿⢸⡆⠀⠀⠀⠀⠀⠀⢻⡆⠀⠀⠀⠀⠀⠀⠀⠀⣿⡿⠟⠋⠉⠁⠀⠀⠀⠀⠀⢸⠁⠀⠀⠀⠀⠀⠀⠀⠀⢸⠇⠀⠀⠀⠀⠀⠀⢸⡇⠈⣟⠀⠈⣷⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⢰⡟⠀⢰⡇⠘⡇⠀⠀⠀⠀⠀⠘⠀⣷⠀⠀⠀⠀⠀⠀⠀⠀⣿⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⠀⠀⠀⠀⠀⠀⠀⠀⠀⡿⠆⠀⠀⠀⠀⠀⠀⣼⠁⠀⢻⡀⠀⠸⣇⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⣼⠁⠀⣼⠁⠀⣿⠀⠀⠀⠀⠀⠀⠀⠻⣇⣀⠀⠀⠀⠀⠀⠀⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢸⡀⠀⠀⠀⠀⠀⠀⠀⣰⡇⠀⠀⠀⠀⠀⠀⠀⣿⡆⠀⠈⣷⡀⠀⢻⡄⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⢰⡇⠀⢀⡏⠀⠀⢻⠀⠀⠀⠀⠀⠀⠀⠀⢻⣦⣄⠀⠀⠀⠀⣠⡟⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣧⠀⠀⠀⠀⠀⣠⣾⣿⠀⠀⠀⠀⠀⠀⢠⣠⡇⡇⠀⠀⠸⣯⡀⠀⢷⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⡾⠀⠀⣼⠁⠀⠀⠸⡇⠀⠀⠀⠀⠀⠀⠀⠀⢿⣽⡧⠴⢶⣿⣿⠖⠒⠛⠛⠃⠀⠀⠀⠚⠋⠉⠉⠉⠙⠓⠲⢾⡛⠻⣽⠃⠀⠀⠀⠀⠀⠀⢸⣿⣃⠀⠀⠀⠀⢹⣷⠀⠘⣧⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⣸⠃⠀⢸⡏⠀⠀⠀⢀⣿⠀⠀⠀⠀⢀⣀⠀⠈⠈⢷⠖⠚⠋⠁⢹⡇⠀⢀⣴⠶⠶⢦⣄⣀⣤⡶⠶⠤⠤⠤⠶⠾⠇⢰⡏⠀⠀⠀⠀⠀⠀⠻⣿⣿⣤⡀⠀⠀⠀⠀⢿⣀⠀⠸⣆⠀⠀⠀
⠀⠀⠀⠀⠀⢰⠏⠀⢀⡿⠀⠀⠀⣰⠟⢹⣿⡄⠀⠀⠀⠻⣄⠀⠀⠘⣷⣤⣄⣀⣈⡙⠛⢹⡷⢶⣦⣴⣾⣛⣻⢯⣴⣦⠴⠖⠃⠀⢀⡾⠀⠀⠀⠀⠀⠀⠀⢰⣿⡇⠀⠹⣆⠀⠀⠸⡞⣧⣆⠀⠹⣄⠀⠀
⠀⠀⠀⠀⢠⡟⠀⠀⡼⠁⠀⠀⣰⠏⠀⠈⣟⣧⠀⠀⠀⠀⢻⣆⠀⠀⠈⣧⡿⠀⠈⠉⠛⠛⣻⣿⡿⢿⣿⡍⠉⠀⠀⠀⠀⠀⠀⠀⡾⠁⠀⠀⠀⢀⣴⠀⠀⡾⣿⠁⠀⠀⠘⣧⠀⠀⢿⠘⣟⠀⠀⢻⡄⠀
⠀⠀⢀⣠⡟⠀⢀⡾⠃⠀⠀⣰⡏⠀⠀⠀⣻⠘⣧⠀⠀⠀⠀⢻⣷⡄⠀⠘⢷⡀⠀⠀⠀⠀⠩⣉⠁⠈⣛⡁⠀⠀⠀⠀⠀⠀⣀⣾⠁⠀⠀⣀⣠⣟⠁⠀⣠⣤⡟⠀⠀⠀⠀⠘⣧⡄⠘⡇⠙⣇⠀⠀⠻
*/
#include <bits/stdc++.h>
#define i64 long long
#define ld long double
#define bit(n,i) ((n>>i)&1)
#define mp make_pair
#define pii pair<int,int>
#define sz(x) (int)((x).size())
#define FOR(i,a,b) for(int i=a; i<=b; i++)
#define FOD(i,a,b) for(int i=a; i>=b; i--)
#define fi first
#define se second
#define pb push_back
#define all(x) x.begin(),x.end()
#define __sumairu__ main()
#define i_love_flandre_scarlet void seggs()
#define file(name) if(fopen(name".inp","r")){freopen(name".inp","r",stdin);freopen(name".out","w",stdout);}
#define brute(name) if(fopen(name".inp","r")){freopen(name".inp","r",stdin);freopen(name".ans","w",stdout);}
#define TIME (1.0*clock()/CLOCKS_PER_SEC)
#define FAST ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ai(n) array<int,n>
using namespace std;
//const int MOD=998244353;
const int MOD=1e9+7;
const int inf=1e9;
mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());

///check the limits, dummy
const int N=5003;
const int C=2;

int T[N*N][C],sum[N*N],cnt[N*N];
bool ok[N][N];

i_love_flandre_scarlet
{
    string s;
    cin>>s;
    int k;
    cin>>k;

    int n=sz(s);
    s=' '+s;

    FOD(l,n,1)
    {
        FOR(r,l,n)if(s[l]==s[r])
        {
            if(r-l<=4)ok[l][r]=1;
            else ok[l][r]=ok[l+2][r-2];
        }
    }

    memset(T,-1,sizeof T);
    int sz=0;
    FOR(l,1,n)
    {
        int v=0;
        FOR(r,l,n)
        {
            if(T[v][s[r]-'a']==-1)T[v][s[r]-'a']=++sz;

            v=T[v][s[r]-'a'];

            cnt[v]+=ok[l][r];
        }
    }

    FOD(i,sz,0)
    {
        sum[i]=cnt[i];

        if(~T[i][0])sum[i]+=sum[T[i][0]];
        if(~T[i][1])sum[i]+=sum[T[i][1]];
    }

    int v=0;
    while(k>cnt[v])
    {
        k-=cnt[v];
        if(T[v][0]==-1)
        {
            cout<<'b';
            v=T[v][1];
        }
        else if(T[v][1]==-1)
        {
            cout<<'a';
            v=T[v][0];
        }
        else if(sum[T[v][0]]>=k)
        {
            cout<<'a';
            v=T[v][0];
        }
        else
        {
            k-=sum[T[v][0]];
            cout<<'b';
            v=T[v][1];
        }
    }
}

__sumairu__
{
    FAST
    file("maybe it's nice to believe in something");

    int t=1;
    //cin>>t;
    while(t--)
    {
        seggs();
    }
    cerr<<"\nTime elapsed: "<<TIME<<" s.\n";
}
/**
abbabaab
7
*/