#include <bits/stdc++.h>
using namespace std;
using namespace std;
vector<int> adj[100005];
vector<int>::iterator it;
int con[100005];
bool visited[100005];
int bfs(int root) {
  int j, i, u, v, mn = 10000000, temp;
  queue<int> Q;
  Q.push(root);
  visited[root] = true;
  while (!Q.empty()) {
    u = Q.front();
    Q.pop();
    for (i = 0; i < adj[u].size(); i++) {
      v = adj[u][i];
      if (!visited[v]) {
        Q.push(v);
        visited[v] = true;
      } else {
        for (j = 0; j < adj[u].size(); j++) {
          if (adj[u][j] != v) {
            it = lower_bound(adj[v].begin(), adj[v].end(), adj[u][j]);
            if (it != adj[v].end() && ((*it) == adj[u][j])) {
              temp = con[u] + con[v] + con[adj[u][j]] - 6;
              if (temp < mn) mn = temp;
            }
          }
        }
      }
    }
  }
  return mn;
}
int main() {
  int j, t, i, t1, t2, n, m, ans;
  t = 1;
  while (t--) {
    scanf("%d", &n);
    scanf("%d", &m);
    memset(visited, 0, sizeof(visited));
    for (i = 0; i < m; i++) {
      scanf("%d%d", &t1, &t2);
      adj[t1].push_back(t2);
      adj[t2].push_back(t1);
    }
    for (i = 1; i <= n; i++) {
      sort(adj[i].begin(), adj[i].end());
    }
    for (i = 1; i <= n; i++) {
      con[i] = adj[i].size();
    }
    ans = 10000000;
    for (i = 1; i <= n; i++) {
      if (!visited[i]) {
        t1 = bfs(i);
        if (t1 < ans) ans = t1;
      }
    }
    if (ans == 10000000)
      printf("-1\n");
    else
      printf("%d\n", ans);
  }
  return 0;
}
