#include <bits/stdc++.h>
using namespace std;
void in() { return; }
template <typename T, typename... Types>
void in(T &a, Types &...b) {
  cin >> (a);
  in(b...);
}
void o() { return; }
template <typename T, typename... Types>
void o(T a, Types... b) {
  cout << (a);
  cout << ' ';
  o(b...);
}
bool sortin(const pair<long long int, long long int> &e,
            const pair<long long int, long long int> &f) {
  return (e.first < f.first);
}
bool POT(long long int x) { return x && (!(x & (x - 1))); }
long long int i, j, k, l, m, n, p, q, r, a, b, c, x, y, z, ts, mn = 1e18,
                                                               mod = 1e9 + 7;
long long int ar[250005], br[250005], xr[250005], tem[250005];
vector<vector<long long int> > v;
int main() {
  {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
  };
  in(n, m);
  v.resize(n + 3, vector<long long int>(n + 3, 0));
  for (long long int i = 0; i <= m - 1; i++) {
    in(a, b);
    xr[a]++;
    xr[b]++;
    ar[i] = a;
    br[i] = b;
    v[a][b] = v[b][a] = 1;
  }
  for (long long int i = 1; i <= n; i++) {
    for (long long int j = 0; j <= m - 1; j++) {
      a = ar[j];
      b = br[j];
      if (v[a][i] and v[b][i]) mn = min(mn, xr[a] + xr[b] + xr[i] - 6);
    }
  }
  if (mn == 1e18) mn = -1;
  o(mn);
}
