#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using mint = modint998244353;
using ll = long long;
using lb = long double;
using VI = vector<int>;
using VL = vector<ll>;
using P = pair<int, int>;
inline void scan(){}
template<class Head,class... Tail>
inline void scan(Head&head,Tail&... tail){std::cin>>head;scan(tail...);}
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define LB(...) lb __VA_ARGS__;scan(__VA_ARGS__)
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
template<class ll> inline bool chmax(ll& a, ll b) { if (a < b) { a = b; return 1; } return 0; }template<class ll> inline bool chmin(ll& a, ll b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T>
std::istream &operator>>(std::istream&is,std::vector<T>&v){for(T &in:v){is>>in;}return is;}
template<typename T>
std::ostream &operator<<(std::ostream&os,const std::vector<T>&v){for(auto it=std::begin(v);it!=std::end(v);){os<<*it<<((++it)!=std::end(v)?" ":"");}return os;}
#define rep(i, s, n) for (int i = (s); i < (int)(n); i++)
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a)  (a).rbegin(),(a).rend()
const int inf = INT_MAX / 2; const ll infl = 1LL << 60;
//_
int main(){
  INT(n, m);
  vector<vector<int>> g(n);
  rep(i, 0, m){
    INT(u, v);
    u--;
    v--;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  vector<int> dist(n, inf);
  queue<int> q;
  dist[0] = 0;
  q.push(0);
  while(!q.empty()){
    int v = q.front();
    q.pop();
    for(auto nv : g[v]){
      if(dist[nv] == inf){
        dist[nv] = dist[v] + 1;
        q.push(nv);
      }
    }
  }
  ll ans = dist[n - 2] + dist[n - 1];
  dist = vector<int>(n, inf);
  dist[n - 2] = 0;
  q.push(n - 2);
  while(!q.empty()){
    int v = q.front();
    q.pop();
    if(v == 0){
      continue;
    }
    for(auto nv : g[v]){
      if(dist[nv] == inf){
        dist[nv] = dist[v] + 1;
        q.push(nv);
      }
    }
  }
  ans += dist[n - 1];
  if(ans >= inf){
    cout << -1 << endl;
  }else{
    cout << ans << endl;
  }
}