#include <bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long; // -2^63 ~ 2^63-1 (9.2*10^18)
using ull = unsigned long long; // 0 ~ 2^64-1 (1.8*10^19)
using ld = long double;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, m, n) for (int i = (m); (i) < int (n); i++)
#define all(v) v.begin(), v.end()
#define bit(n) (1ll<<(n)) // 2^n
#define sz(x) ((int)(x).size())
#define fi first
#define se second
template<class T> using maxpq = priority_queue<T>;
template<class T> using minpq = priority_queue<T, vector<T>, greater<T>>;
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; }
const ll llINF = 1000000000000000000;
void YesNo(bool flag){
    if(flag) cout << "Yes" << endl;
    else cout << "No" << endl;
}
// 多次元vector生成
template<ll idx = 0, ll n, class T>
auto make_vec(const ll (&d)[n], T&& init) noexcept {
    if constexpr (idx < n) return std::vector(d[idx], make_vec<idx + 1>(d, std::forward<T>(init)));
    else return init;
}
template<class T, ll idx = 0, ll n>
auto make_vec(const ll (&d)[n], const T& init = {}) noexcept {
    if constexpr (idx < n) return std::vector(d[idx], make_vec<idx + 1>(d, init));
    else return init;
}
// auto 変数名 = make_vec<型>({a, b, ...}, 初期値);

int main(){
    int N, M;
    cin >> N >> M;
    vector<vector<int>> G(N);
    rep(i, M){
        int u, v;
        cin >> u >> v;
        u--, v--;
        G[u].push_back(v);
    }

    vector<vector<int>> s = {{0, N-2, N-1}, {0, N-1, N-2}};
    vector<vector<int>> g = {{N-2, N-1, 0}, {N-1, N-2, 0}};

    vector<int> ans(2, 1e7);
    rep(i, 2){
        int tmp = 0;
        rep(j, 3){
            vector<int> dist(N, -1); // 全頂点を「未訪問」に初期化
            queue<int> que;

            dist[s[i][j]] = 0;
            que.push(s[i][j]);

            while (!que.empty()) {
                int v = que.front(); // キューから先頭頂点を取り出す
                que.pop();

                // v から辿れる頂点をすべて調べる
                for (int nv : G[v]) {
                    if (dist[nv] != -1) continue; // すでに発見済みの頂点は探索しない

                    // 新たな白色頂点 nv について距離情報を更新してキューに追加する
                    dist[nv] = dist[v] + 1;
                    que.push(nv);
                }
            }

            if(dist[g[i][j]] == -1){
                break;
            }
            else{
                tmp += dist[g[i][j]];
                //cout << dist[g[i][j]] << " ";
                if(j == 2){
                    ans[i] = tmp;
                    //cout << endl;
                }
            }
        }
    }

    if(ans[0] == 1e7 && ans[1] == 1e7){
        cout << -1 << endl;
    }
    else{
        cout << min(ans[0], ans[1]) << endl;
    }

    //cout << ans[0] << " " << ans[1] << endl;
}