#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
using namespace std;
const int inf = 100000000;
vector<int> bfs(int s, int N, vector<vector<int>> &G) {
  vector<int> dist(N, inf);
  dist[s] = 0;
  queue<int> que;
  que.push(s);
  while(!que.empty()) {
    int u = que.front();
    que.pop();
    for(int v : G[u]) {
      if(dist[v] == inf) {
        dist[v] = dist[u] + 1;
        que.push(v);
      }
    }
  }
  return dist;
}
int main() {
  cin.tie(0); cout.tie(0);
  ios::sync_with_stdio(false);
  int N, M;
  cin >> N >> M;
  vector<vector<int>> G(N);
  while(M--) {
    int u, v;
    cin >> u >> v;
    G[--u].emplace_back(--v);
  }
  vector<int> d0 = bfs(0, N, G);
  vector<int> d1 = bfs(N - 2, N, G);
  vector<int> d2 = bfs(N - 1, N, G);
  int ans = min(d0[N - 2] + d1[N - 1] + d2[0], d0[N - 1] + d2[N - 2] + d1[0]);
  cout << (ans >= inf ? -1 : ans) << '\n';
  return 0;
}