Boilerplate for layout tasks

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Display six colored blocks on the screen without any extra margins:

Use flexbox
Each block should be 300px high.
Each block should have its number (1 to 6) placed in its center in white Arial 100px font.
- Block 1 must be red
- Block 2 must be 4/5 red and 1/5 black
- Block 3 must be 3/5 red and 2/5 black
- Block 4 must be 2/5 red and 3/5 black
- Block 5 must be 1/5 red and 4/5 black
- Block 6 must be black.
The width of each block should be not less than its height (300px)
Blocks must be put in 2 to 6 rows and stretched: each row must fit either 1, 2, or 3 blocks. See the screenshots here.
The first row is to be read from left ro right, the second row from right to left, the third row again from left to right (this way we have a zigzag, or a snake).

3 columns	2columns	1 column