#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cout << x; }
void __print(long x) { cout << x; }
void __print(long long x) { cout << x; }
void __print(unsigned x) { cout << x; }
void __print(unsigned long x) { cout << x; }
void __print(unsigned long long x) { cout << x; }
void __print(float x) { cout << x; }
void __print(double x) { cout << x; }
void __print(long double x) { cout << x; }
void __print(char x) { cout << '\'' << x << '\''; }
void __print(const char *x) { cout << '\"' << x << '\"'; }
void __print(const string &x) { cout << '\"' << x << '\"'; }
void __print(bool x) { cout << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cout << '{';
  __print(x.first);
  cout << ',';
  __print(x.second);
  cout << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cout << '{';
  for (auto &i : x) cout << (f++ ? "," : ""), __print(i);
  cout << "}";
}
void _print() { cout << "\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cout << ", ";
  _print(v...);
}
const int mod = 1e9 + 7, mxn = 401;
vector<int> dt(mxn, mod), db(mxn, mod);
vector<bool> vist(mxn, 0), visb(mxn, 0);
int t[mxn + 1][mxn + 1];
int solve() {
  int n, e;
  cin >> n >> e;
  for (long long i = (0); i < (e); ++i) {
    int x, y;
    cin >> x >> y;
    t[x][y] = 1;
    t[y][x] = 1;
  }
  queue<int> qt, qb;
  qt.push(1);
  vist[1] = 1;
  dt[1] = 0;
  qb.push(1);
  visb[1] = 1;
  db[1] = 0;
  while (qt.size()) {
    int cur = qt.front();
    qt.pop();
    for (long long i = (1); i < (n + 1); ++i) {
      if (vist[i] == 0 and t[cur][i]) {
        qt.push(i);
        vist[i] = 1;
        dt[i] = min(dt[i], dt[cur] + 1);
      }
    }
  }
  while (qb.size()) {
    int cur = qb.front();
    qb.pop();
    for (long long i = (1); i < (n + 1); ++i) {
      if (visb[i] == 0 and !t[cur][i]) {
        qb.push(i);
        visb[i] = 1;
        db[i] = min(db[cur] + 1, db[i]);
      }
    }
  }
  int ans = max(dt[n], db[n]);
  if (ans == mod) ans = -1;
  cout << ans << "\n";
  return 0;
}
int main() {
  ios_base::sync_with_stdio(NULL);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
