#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main() {
int n,m; cin>>n>>m;
vector<pair<int,int>> doges;
for (int i=0; i<m; i++) {
int b,p; cin>>b>>p;
doges.push_back({b,p});
}
vector<vector<pair<int,ll>>> al(m,vector<pair<int,ll>>());
for (int i=0; i<m; i++) {
if (i==1) continue;
for (int j=0; j<m; j++) {
if (i==j) continue;
int d=abs(doges[i].first-doges[j].first);
if (d%doges[i].second==0) {
al[i].push_back({j,d/doges[i].second});
}
}
}
vector<ll> dist(m,1e18);
dist[0]=0;
priority_queue<pair<ll,int>,vector<pair<ll,int>>,greater<>> p;
p.push({0ll,0});
while (!p.empty()) {
int t=p.top().second; p.pop();
for (auto v: al[t]) {
if (dist[t]+v.second<dist[v.first]) {
dist[v.first]=dist[t]+v.second;
p.push({dist[v.first],v.first});
}
}
}
if (dist[1]==1e18) cout<<-1;
else cout<<dist[1];
return 0;
}
