#include <bits/stdc++.h>
#define rep(i, l, r) for (auto i = (l); i < (r); i++)
#define chmin(dest, src) if ((dest) > (src)) dest = (src)
#define chmax(dest, src) if ((dest) < (src)) dest = (src)
using namespace std;
using ll = long long;
const ll INF = 1e9;
int main() {
  int N, M; cin >> N >> M;
  vector<vector<int>> g(N, vector<int>()) ;
  rep (i, 0, M) {
    int a, b; cin >> a >> b; a--, b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  vector<ll> dist(N, INF);
  dist[0] = 0;
  queue<int> q;
  q.push(0);
  while (!q.empty()) {
    int u = q.front(); q.pop();
    if (u == N - 1) {
      cout << dist[u] << endl;
      exit(0);
    }
    for (auto v : g[u]) {
      if (dist[v] < INF) continue;
      dist[v] = dist[u] + 1;
      q.push(v);
    }
  }
  cout << -1 << endl;
  return 0;
}