#include <bits/stdc++.h>
using namespace std;
bitset<4001> g[4001];
int main() {
  int n, m;
  cin >> n >> m;
  vector<pair<int, int> > v;
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a][b] = 1;
    g[b][a] = 1;
    v.push_back(pair<int, int>(a, b));
  }
  int minx = 100000000;
  bool was = 0;
  for (int i = 0; i < v.size(); i++) {
    int a = v[i].first;
    int b = v[i].second;
    for (int j = 0; j < n; j++) {
      if (g[a][j] && g[b][j]) {
        minx = min(minx, (int)(g[a].count() + g[b].count() + g[j].count()) - 6);
        was = true;
      }
    }
  }
  if (was)
    cout << minx;
  else
    cout << -1;
  return 0;
}
