#include<iostream>
#include<vector>
#include<queue>
using namespace std;
int main(){
int N, M; cin >> N >> M;
vector<vector<int> >adj(N+1);
vector<bool> visit(N + 1, false);
for (int i = 0; i < M; i++) {
int u, v; cin >> u >> v;
adj[u].push_back(v);
}
priority_queue <pair<int, pair<int, int> > >pq;
pq.push(make_pair(0, make_pair(0, 1)));
while (true){
if (pq.empty()){
cout << -1 << endl;
break;
}
int val = -pq.top().first;
int u = pq.top().second.first;
int v = pq.top().second.second;
pq.pop();
for (int i = 0; i < adj[v].size(); i++){
if (!visit[adj[v][i]]) {
if (adj[v][i] == N) {
cout << val + 1 << endl;
return 0;
}
visit[adj[v][i]] = true;
pq.push(make_pair(-1 * (val + 1), make_pair(v, adj[v][i])));
}
}
}
return 0;
}
