#include <iostream>
#include <cstdio>
#include <vector>
#include <queue>
using namespace std;
vector<vector<int> > g;
int find(int n){
vector<int> dist(n+1, 0x7fffffff);
dist[1] = 0;
priority_queue<pair<int,int> >pq;
pq.push(make_pair(0, 1));
while(!pq.empty() ) {
int cost = -pq.top().first;
int here = pq.top().second;
pq.pop();
if(cost > dist[here]) {
continue;
}
for(int i=0;i<g[here].size();i++) {
int there = g[here][i];
int ndist = cost+1;
if(dist[there] > ndist) {
dist[there] = ndist;
pq.push(make_pair(-ndist, there));
}
}
}
return dist[n];
}
int main() {
int n, m;
scanf("%d %d", &n, &m);
if(n==1) {
printf("0\n");
return 0;
}
g.resize(n+1);
// make graph
for (int i=m;i!=0;i--) {
int x,y;
scanf("%d %d", &x, &y);
g[x].push_back(y);
}
// find solution
int result = find(n);
if(result == 0x7fffffff){
printf("-1\n");
}
else {
printf("%d\n",result);
}
return 0;
}
