#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); ++i)

int main() {
  int n, m;
  cin >> n >> m;
  vector Graph(n, vector<int>(0));
  rep(_, m) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    Graph[a].push_back(b);
  }
  constexpr int INF = 1e8;
  auto bfs = [&](int s, int g) {
    vector<int> dist(n, INF);
    queue<int> q;
    q.push(s);
    dist[s] = 0;
    while (!q.empty()) {
      int v = q.front();
      q.pop();
      for (int u : Graph[v]) {
        if (dist[u] != INF) continue;
        dist[u] = dist[v] + 1;
        q.push(u);
      }
    }
    return dist[g];
  };
  int d_0n = bfs(0, n - 1);
  int d_n0 = bfs(n - 1, 0);
  int d_0n1 = bfs(0, n - 2);
  int d_n10 = bfs(n - 2, 0);
  int d_nn1 = bfs(n - 1, n - 2);
  int d_n1n = bfs(n - 2, n - 1);
  int ans = INF;
  ans = min(ans, d_0n + d_nn1 + d_n10);
  ans = min(ans, d_0n1 + d_n1n + d_n0);
  Debug(d_0n, d_n0, d_0n1, d_n10, d_nn1, d_n1n);
  cout << (ans == INF ? -1 : ans) << endl;
  return 0;
}