#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  vector<vector<int>> G(N);
  for (int i = 0; i < M; i++) {
    int u, v;
    cin >> u >> v;
    u--, v--;
    G[u].push_back(v);
    // G[v].push_back(u);
  }
  const int inf = 10000000;
  queue<int> que;
  vector<int> dist1(N, inf), dist2(N, inf), dist3(N, inf);
  que.push(0);
  dist1[0] = 0;
  while (!que.empty()) {
    int cur = que.front();
    que.pop();
    for (auto nxt : G[cur]) {
      if (dist1[nxt] > dist1[cur] + 1) {
        que.push(nxt);
        dist1[nxt] = dist1[cur] + 1;
      }
    }
  }
  que.push(N - 2);
  dist2[N - 2] = 0;
  while (!que.empty()) {
    int cur = que.front();
    que.pop();
    for (auto nxt : G[cur]) {
      if (dist2[nxt] > dist2[cur] + 1) {
        que.push(nxt);
        dist2[nxt] = dist2[cur] + 1;
      }
    }
  }
  que.push(N - 1);
  dist3[N - 1] = 0;
  while (!que.empty()) {
    int cur = que.front();
    que.pop();
    for (auto nxt : G[cur]) {
      if (dist3[nxt] > dist3[cur] + 1) {
        que.push(nxt);
        dist3[nxt] = dist3[cur] + 1;
      }
    }
  }
  int ans = min(dist1[N - 2] + dist2[N - 1] + dist3[0], dist1[N - 1] + dist3[N - 2] + dist2[0]);
  if (ans >= inf) {
    ans = -1;
  }
  cout << ans << endl;
  return 0;
}

// iku junban
// 1 -> N - 1 -> N -> 1
// 1 -> N -> N - 1 -> 1