#include <bits/stdc++.h>
#include <atcoder/all>
using ll = long long;
#define MOD 1000000007
#define Mod 998244353
const int MAX = 1000000005;
const long long INF = 1000000000000000005LL;
using namespace std;
using namespace atcoder;



int main() {
    ios::sync_with_stdio(0);cin.tie();
    int N, M;
    cin >> N >> M;
    vector<vector<int>> G(N);
    for (int i = 0; i < M; i++) {
        int u, v;
        cin >> u >> v;
        u--; v--;
        G[u].push_back(v);
    }
    vector<vector<int>> dist(3, vector<int>(N, -1));
    for (int j = 0; j < 3; j++) {
        queue<int> que;
        if (j == 0) {dist[j][0] = 0; que.push(0);}
        else if (j == 1) {dist[j][N-2] = 0; que.push(N-2);}
        else {dist[j][N-1] = 0; que.push(N-1);}
        while (que.size()) {
            int v = que.front();
            que.pop();
            for (auto &nv : G[v]) {
                if (dist[j][nv] != -1) continue;
                dist[j][nv] = dist[j][v] + 1;
                que.push(nv);
            }
        }
    }
    int ans = -1;
    if (dist[0][N-1] != -1 && dist[2][N-2] != -1 && dist[1][0] != -1) {
        ans = dist[0][N-1] + dist[2][N-2] + dist[1][0];
    }
    if (dist[0][N-2] != -1 && dist[1][N-1] != -1 && dist[2][0] != -1) {
        int d = dist[0][N-2] + dist[1][N-1] + dist[2][0];
        ans = (ans == -1 ? d : min(ans, d));
    }
    cout << ans << endl;
}