#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
// long long
using ll = long long;
// unsigned long long 
using ull = unsigned long long;
// string 
using str = string;
// long double
using ld = long double;
// pair 
using pll = pair < ll, ll >;
// tuple
using tlll = tuple < ll, ll, ll >;
// vector
template < class T > using v = vector < T >;
using vl = v < ll >;
using vbl = v < bool >;
// priority_queue 
template < class T > using pq = priority_queue < T >;
template < class T > using pqg = priority_queue < T, v < T >, greater < T > >;
// unordered_set
template < class T > using uset = unordered_set < T > ;
// unordered_map
template < class T, class U > using umap = unordered_map < T, U > ;
// rep
#define rep(i, a, b) for(ll i = a; i <= b; i ++)
#define Rep(i, a) for(ll i = 0; i < a; i ++)
#define rrep(i, a, b) for(ll i = a; i >= b; i --)
#define repp(i, a, b, c) for(ll i = a; i <= b; i += c)
// binary search
#define lobo(x, k) lower_bound(x.begin(), x.end(), k) - x.begin()
#define Lobo(x, a, b, k) lower_bound(x + a, x + b + 1, k) - x
#define upbo(x, k) upper_bound(x.begin(), x.end(), k) - x.begin()
#define Upbo(x, a, b, k) upper_bound(x + a, x + b + 1, k) - x;
// snippet
#define eruni(x) x.erase(unique(x.begin(), x.end()), x.end())
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define all(x) x.begin(), x.end()
#define rll(x) x.rbegin(), x.rend()
#define sz(x) (ll)x.size()
#define f first 
#define s second 
#define in insert
#define pf push_front
#define p_f pop_front
#define pb push_back
#define p_b pop_back
#define p_p prev_permutation
#define n_p next_permutation
#define led leader
#define print(a) printf("%.20Lf", a)
#define func function
// function
template < class T > bool chmax(T &a, const T &b) {if (a < b) {a = b; return 1;} return 0;}
template < class T > bool chmin(T &a, const T &b) {if (b < a) {a = b; return 1;} return 0;}
// out
void Yes(bool f = true) {cout << (f ? "Yes" : "No" ) << endl;}
void No(bool f = true) {cout << (f ? "No" : "Yes" ) << endl;}
void YES(bool f = true) {cout << (f ? "YES" : "NO" ) << endl;}
void NO(bool f = true) {cout << (f ? "NO" : "YES" ) << endl;}
template < class T > void out(T a) {cout << a << endl;}
template < class T > void out1(T a) {cout << a << " ";}
template < class T, class U > void out2(T a, U b) {cout << a << " " << b << endl;}
template < class T> void allout(T &a) {
   for(auto x : a) cout << x << " ";
   cout << endl;
}
template < class T > void allout1(T &a) {
   for(auto x : a) cout << x << endl;
}

// modint
using modi = modint998244353;
using mint = modint1000000007;
// constant
const ll md = 998244353;
const ll mo = 1000000007;
const ld pi = acos(-1);
const ll inf = (1LL << 60);
const ll n1 = 100009;
const ll n2 = 200009;
const ll n3 = 300009;
const ll n5 = 500009;
// grid
const pll ar1[4] = { {0, 0}, {1, 0}, {0, 1}, {1, 1} };
const pll ar2[4] = { {-1, 0}, {0, -1}, {1, 0}, {0, 1} };
const pll ar3[8] = { {-1, 0}, {0, -1}, {1, 0}, {0, 1}, {-1, 1}, {1, -1}, {1, 1}, {-1, -1} };

int main(){
   ll n, m;
   vl G[n2];
   cin >> n >> m;
   Rep(i, m){
      ll a, b;
      cin >> a >> b;
      G[a].eb(b);
   }
   auto bfs = [&](ll x, ll y){
      vl D(n2, -1);
      queue < ll > Q;
      Q.push(x);
      D[x] = 0;
      while(sz(Q)){
         ll p = Q.front();
         Q.pop();
         for(auto nex : G[p]){
            if(D[nex] == -1){
               D[nex] = D[p] + 1;
               Q.push(nex);
            }
         }
      }
      return D[y];
   };
   ll d1 = -1, d2 = -1, ans = inf;
   bool f1 = true, f2 = true;
   if(bfs(1, n - 1) == -1 || bfs(n - 1, n) == -1 || bfs(n, 1) == -1) f1 = false;
   if(bfs(1, n) == -1 || bfs(n, n - 1) == -1 || bfs(n - 1, 1) == -1) f2 = false;
   if(f1){
      d1 ++;
      d1 += bfs(1, n - 1);
      d1 += bfs(n - 1, n);
      d1 += bfs(n, 1);
      chmin(ans, d1);
   }
   if(f2){
      d2 ++;
      d2 += bfs(1, n);
      d2 += bfs(n, n - 1);
      d2 += bfs(n - 1, 1);
      chmin(ans, d2);
   }
   if(ans == inf) ans = -1;
   out(ans);
}