#include <bits/stdc++.h>
using namespace std;
const int MAX = (3e4)+1, INF = 1e9;
int n, m, s, e, a[MAX], b[MAX], d[MAX];
vector<int> ls[MAX];
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
for(int i = 0; i < n; i++) d[i] = INF;
for(int i = 0; i < m; i++) {
cin >> a[i] >> b[i];
ls[a[i]].push_back(b[i]);
if(i == 0) s = a[i];
if(i == 1) e = a[i];
}
for(int i = 0; i < n; i++) {
sort(ls[i].begin(), ls[i].end());
ls[i].resize(distance(ls[i].begin(), unique(ls[i].begin(), ls[i].end())));
}
priority_queue<pair<int,int>, vector<pair<int,int> >, greater<pair<int,int> > > pq;
pq.emplace(d[s] = 0, s);
while(!pq.empty()) {
int dd, u; tie(dd, u) = pq.top();
pq.pop();
if(d[u] < dd) continue;
for(int p: ls[u]) {
for(int i = 1; u + i*p < n; i++) {
if(d[u] + i < d[u + i*p]) pq.emplace(d[u + i*p] = d[u] + i, u + i*p);
// if(binary_search(ls[u + i*p].begin(), ls[u + i*p].end(), p)) break;
}
for(int i = 1; u - i*p >= 0; i++) {
if(d[u] + i < d[u - i*p]) pq.emplace(d[u - i*p] = d[u] + i, u - i*p);
// if(binary_search(ls[u - i*p].begin(), ls[u - i*p].end(), p)) break;
}
}
}
cout << (d[e] == INF ? -1 : d[e]);
}
