#include <bits/stdc++.h>

using namespace std;
using ll = long long;

vector<ll> bfs(vector<vector<int>> &E, int start){

    int N = E.size();
    int from, to;
    queue<int> que;
    vector<ll> dist(N, 1e9);

    dist[start] = 0;
    que.push(start);

    while(!que.empty()){
        from = que.front();
        que.pop();
        for (auto to : E[from]){
            if (dist[to] != 1e9) continue;
            dist[to] = dist[from] + 1;
            que.push(to);
        }
    }

    return dist;
}

int main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    int N, M, A, B;
    ll ans;
    cin >> N >> M;
    vector<vector<int>> E(N);

    for (int i=0; i<M; i++){
        cin >> A >> B; A--; B--;
        E[A].push_back(B);
    }

    vector<ll> dist1 = bfs(E, 0), dist2 = bfs(E, N-2), dist3 = bfs(E, N-1);
    ans = min(dist1[N-2]+dist2[N-1]+dist3[0], dist1[N-1]+dist3[N-2]+dist2[0]);

    cout << (ans < 1e9 ? ans : -1) << endl;

    return 0;
}