#include <bits/stdc++.h>
//#include<atcoder/all>
using namespace std;
//using namespace atcoder;
using ll = long long;
const ll INF = 1000000000000000000LL;
//const ll mod = 998244353;
int n, m, u[100009], v[100009];
vector<int>G[100009];
ll dist[3][100009];
queue<int>Q;
int main() {
	cin >> n >> m;
	for (int i = 1; i <= m; i++) {
		cin >> u[i] >> v[i];
		G[u[i]].push_back(v[i]);
	}
	for (int i = 0; i < 3; i++) {
		for (int j = 1; j <= n; j++)dist[i][j] = -1;
	}
	dist[0][1] = 0;
	Q.push(1);
	while (!Q.empty()) {
		int pos = Q.front(); Q.pop();
		for (int i = 0; i < G[pos].size(); i++) {
			int nex = G[pos][i];
			if (dist[0][nex] == -1) {
				dist[0][nex] = dist[0][pos] + 1;
				Q.push(nex);
			}
		}
	}
	dist[1][n - 1] = 0;
	Q.push(n - 1);
	while (!Q.empty()) {
		int pos = Q.front(); Q.pop();
		for (int i = 0; i < G[pos].size(); i++) {
			int nex = G[pos][i];
			if (dist[1][nex] == -1) {
				dist[1][nex] = dist[1][pos] + 1;
				Q.push(nex);
			}
		}
	}
	dist[2][n] = 0;
	Q.push(n);
	while (!Q.empty()) {
		int pos = Q.front(); Q.pop();
		for (int i = 0; i < G[pos].size(); i++) {
			int nex = G[pos][i];
			if (dist[2][nex] == -1) {
				dist[2][nex] = dist[2][pos] + 1;
				Q.push(nex);
			}
		}
	}
	ll ans1 = INF, ans2 = INF, ans3 = INF;
	if (dist[0][n - 1] != -1 && dist[1][n] != -1 && dist[2][1] != -1) {
		ans1 = dist[0][n - 1] + dist[1][n] + dist[2][1];
	}
	if (dist[0][n] != -1 && dist[2][n-1] != -1 && dist[1][1] != -1) {
		ans2 = dist[0][n] + dist[2][n-1] + dist[1][1];
	}
	if (dist[0][n - 1] != -1 && dist[1][1] != -1 && dist[0][n] != -1&&dist[2][1]!=-1) {
		ans3 = dist[0][n - 1] + dist[1][1] + dist[0][n]+dist[2][1];
	}
	//cout << ans1 << " " << ans2 << " " << ans3 << endl;
	ll ans = min(ans1, min(ans2, ans3));
	if (ans != INF)cout << ans << endl;
	else cout << "-1" << endl;
}