#include <bits/stdc++.h>
using namespace std;
const int MAX = (2e3)+1, INF = 1e9;
int n, m, b[MAX], p[MAX], g[MAX][MAX], d[MAX][MAX];
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++) for(int j = 0; j < m; j++) if(i != j) g[i][j] = d[i][j] = INF;
for(int i = 0; i < m; i++) cin >> b[i] >> p[i];
for(int i = 0; i < m; i++) for(int j = 0; j < m; j++) if(i != j) {
if(abs(b[i] - b[j]) % p[i] == 0) g[i][j] = abs(b[i] - b[j]) / p[i];
}
queue<int> q;
q.push(0);
while(!q.empty()) {
int u = q.front();
q.pop();
for(int v = 0; v < m; v++) if(u != v && d[0][v] == INF) {
d[0][v] = d[0][u] + g[u][v];
q.push(v);
}
}
cout << (d[0][1] == INF ? -1 : d[0][1]);
}
