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

using ll = long long;
#define rep(i, n) for(int i = 0; i < n; i++)

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);
	}
	int INF = 1e9;
	vector<int> d(N, INF), d1(N, INF);
	using P = pair<int, int>;
	queue<P> q;
	q.push(P(0, 0));
	d[0] = 0;
	while(q.size()) {
		int u, dd;
		tie(u, dd) = q.front();
		q.pop();
		for(int e : g[u]) {
			if(d[e] > dd + 1) {
				d[e] = dd + 1;
				q.push(P(e, d[e]));
			}
		}
	}
	int ans = max(d[N - 1], d[N - 2]);
	if(ans == INF) {
		cout << -1 << endl;
		return 0;
	}
	int s = N - 1;
	if(d[N - 2] > d[N - 1]) s = N - 2;
	q.push(P(s, 0));
	d1[s] = 0;
	while(q.size()) {
		int u, dd;
		tie(u, dd) = q.front();
		q.pop();
		for(int e : g[u]) {
			if(d1[e] > dd + 1) {
				d1[e] = dd + 1;
				q.push(P(e, d1[e]));
			}
		}
	}
	if(d1[0] == INF) {
		cout << -1 << endl;
		return 0;
	}
	cout << ans + d1[0] << endl;
};