#include <bits/stdc++.h>
#define f first
#define s second
using namespace std;
int d[30300][2];
vector<int> v[30300];
int p[30300];
int e[30303];
int main() {
int n,m;
cin >> n >> m;
for (int i = 0; i < m; i++) {
cin >> d[i][0] >> d[i][1];
int x = d[i][0];
int y = d[i][1];
v[x].push_back(y);
}
priority_queue<pair<int,int>> q;
for (int i = 0; i < 30300; i++)
e[i] = 1e9;
q.push({0,d[0][0]});
e[d[0][0]] = 0;
while(!q.empty()) {
int s = q.top().s;
int x = -q.top().f;
q.pop();
if (x > e[s]) continue;
if (s == d[1][0]) {
cout << x << "\n";
return 0;
}
for (int u : v[s]) {
int it = 1;
for (int i = s-u; i >= 0; i -= u) {
if (e[s] + it < e[i]) {
// cout << s << " -> " << i << " " << it <<"\n";
e[i] = it + e[s];
q.push({-e[i],i});
}
++it;
}
it = 1;
for (int i = s+u; i < n; i += u) {
if (e[s] + it < e[i]) {
// cout << s << " -> " << i << " " << it << "\n";
e[i] = it + e[s];
q.push({-e[i],i});
}
++it;
}
}
}
cout << -1 << "\n";
}
