#include<bits/stdc++.h>
using namespace std;

#define ll long long

const ll INF = 1LL << 60;

vector<ll> bfs(int start, vector<vector<int>> &graph, int N) {
    vector<ll> dists(N, INF);
    dists[start] = 0;

    queue<pair<int, ll>> q;
    q.push(pair<int, ll>(start, 0));

    while (!q.empty()) {
        int now_v = q.front().first;
        ll d = q.front().second;
        q.pop();

        for (int next_v : graph[now_v]) {
            if (dists[next_v] < INF) continue;
            dists[next_v] = d + 1;
            q.push(pair<int, ll>(next_v, d + 1));
        }
    }

    return dists;
}

int main() {
    int N, M;
    cin >> N >> M;

    vector<vector<int>> graph(N, vector<int>());
    for (int i = 0; i < M; i++) {
        int u, v;
        cin >> u >> v;
        u -= 1, v -= 1;
        graph[u].push_back(v);
    }

    vector<ll> dist_0 = bfs(0, graph, N);
    vector<ll> dist_N_2 = bfs(N-2, graph, N);
    vector<ll> dist_N_1 = bfs(N-1, graph, N);

    // 町1 -> 町N-1 -> 町N -> 町1 の最短経路の長さ
    ll d_1 = dist_0[N-2] + dist_N_2[N-1] + dist_N_1[0];

    // 町1 -> 町N -> 町n-1 -> 町1 の最短経路の長さ
    ll d_2 = dist_0[N-1] + dist_N_1[N-2] + dist_N_2[0];

    ll ans = min(d_1, d_2);

    if (ans < INF) cout << ans << endl;
    else cout << -1 << endl;
}