#include <bits/stdc++.h>
using namespace std;
#define int long long
const int inf = 1e18;
signed main(){
ios::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
int n, m;
cin >> n >> m;
vector<int> d(m), s(m);
for(int i = 0; i < m; ++i){
cin >> d[i] >> s[i];
}
assert(m <= 3);
if(m == 2){
int dist = abs(d[0] - d[1]);
if(dist % s[0] == 0){
cout << dist / s[0] << "\n";
}else{
cout << "-1\n";
}
}else{
int dist_ab = abs(d[0] - d[1]);
int dist_ac = abs(d[0] - d[2]);
int dist_cb = abs(d[1] - d[2]);
int ans = inf;
if(dist_ab % s[0] == 0){
ans = min(ans, dist_ab / s[0]);
}
if(dist_ac % s[0] == 0 && dist_cb % s[2] == 0){
ans = min(ans, dist_ac / s[0] + dist_cb / s[2]);
}
if(ans == inf) cout << "-1\n";
else cout << ans << "\n";
}
return 0;
}
