#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pii pair <int, int>
#define fi first
#define se second
#define mp make_pair
const int NM = 30000, MM = 2000, inf = 1e18;
int N, M, B[MM+5], P[MM+5];
int d[MM+5];
priority_queue <pii, vector <pii>, greater <pii> > pq;
void dijkstra(){
for (int i = 0; i < M; i++) d[i] = +inf;
d[0] = 0;
while (!pq.empty()) pq.pop();
pq.push(mp(0, 0));
while (!pq.empty()){
int u = pq.top().se;
if (d[u] != pq.top().fi){
pq.pop();
continue;
}
pq.pop();
for (int v = 0; v < M; v++){
if ((B[u]-B[v])%P[u] != 0) continue;
int w = abs(B[u]-B[v])/P[u];
if (d[u]+w < d[v]){
d[v] = d[u]+w;
pq.push(mp(d[v], v));
}
}
}
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> N >> M;
for (int i = 0; i < M; i++)
cin >> B[i] >> P[i];
dijkstra();
if (d[1] == +inf) cout << -1;
else cout << d[1];
return 0;
}
