boj2644.cpp

#include <bits/stdc++.h>

using namespace std;

int n, k, start, goal;
bool visited[101];

int bfs(vector<vector<int>> &lst){
  queue<pair<int, int>> q;
  q.push({0, start});
  visited[start] = true;

  while(!q.empty()){
    int depth = q.front().first;
    int now = q.front().second;
    q.pop();

    for (int i=0; i<lst[now].size(); i++){
      if (lst[now][i] == goal) return depth + 1;
      else if (!visited[lst[now][i]]){
        q.push({depth + 1, lst[now][i]});
        visited[lst[now][i]] = true;
      }
    }
  }
  return -1;
}

int main(void){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  freopen("input.txt", "r", stdin);

  cin >> n >> start >> goal >> k;
  vector<vector<int>> lst(n + 1, vector<int> ());

  for (int i=0; i<k; i++){
    int u, v;
    cin >> u >> v;
    lst[u].push_back(v);
    lst[v].push_back(u);
  }
  cout << bfs(lst);
}