#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <set>
#include <map>
#include <string>
#include <queue>
using namespace std;
int main() {
  int n, m; cin >> n >> m;
  vector to(n, vector<int>());
  for (int i = 0; i < m; i++) {
    int u, v; cin >> u >> v;
    u--; v--;
    to[u].push_back(v);
  }
  const long long INF = 1001001001;
  auto bfs = [&](int s) -> vector<long long> {
    vector<long long> dp(n, INF);
    queue<int> q;
    q.push(s);
    dp[s] = 0;
    while (q.size()) {
      auto v = q.front(); q.pop();
      for (auto u : to[v]) {
        if (dp[u] == INF) {
          dp[u] = dp[v] + 1;
          q.push(u);
        }
      }
    }
    return dp;
  };
  auto dp0 = bfs(0);
  // for (int i = 0; i < n; i++) {
  //   cout << (dp0[i] == INF ? -1ll : dp0[i]) << (i + 1 == n ? "\n" : " ");
  // }
  auto dpnp = bfs(n - 2);
  auto dpn = bfs(n - 1);
  long long ans = min({dp0[n - 2] + dpnp[n - 1] + dpn[0], dp0[n - 2] + dpnp[0] + dp0[n - 1] + dpn[0], dp0[n - 1] + dpn[n - 2] + dpnp[0]});
  if (ans >= INF) ans = -1;
  cout << ans << endl;

}