#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vi;
typedef pair<ll,ll> pii;
typedef vector<pii> vii;
typedef vector<vii> vvii;
int main(){
ll l,n;
cin >> l >> n;
vii v(n);
for (ll i=0;i<n;i++) cin >> v[i].first >> v[i].second;
vvii conexiones(n);
for (ll i=0;i<n;i++){
pii x=v[i];
for (ll j=0;j<n;j++){
pii y=v[j];
if (i==j) continue;
if (y.first%x.second==x.first%x.second) conexiones[i].push_back(pii{abs(x.first-y.first)/x.second,j});
}
}
priority_queue<pii> cola;
cola.push(pii{0,0});
bool T=true;
vi porvisitar(n,true);
//for (int i=0;i<n;i++){
// for (pii x:conexiones[i]) cerr << x.second<<x.first<<' ';
// cerr<<'\n';
//}
while (cola.size()){
pii x=cola.top();
cola.pop();
if (porvisitar[x.second]){
porvisitar[x.second]=false;
if (x.second==1){
T=false;
cout << -x.first << '\n';
break;
}
for (pii y:conexiones[x.second]){
cola.push(pii{x.first-y.first,y.second});
}
}
}
if (T) cout << -1<<'\n';
}
