#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,m;
int b[30010],p[30010],dp[30010];
int dist[30010];
vector<pair<int,int>> v[30010];
priority_queue<pair<int,int>> pq;
signed main(){
cin>>n>>m;
for(int i=0;i<m;i++){
cin>>b[i]>>p[i];
}
for(int i=0;i<m;i++){
for(int j=0;j<n;j++){
if(i==j)
continue;
if(abs(b[j]-b[i])%p[i]==0){
v[i].push_back({j,abs(b[j]-b[i])/p[i]});
}
}
}
for(int i=1;i<m;i++)
dist[i]=INT_MAX;
pq.push({0,0});
while(pq.empty()==0){
int w=-pq.top().first;
int u=pq.top().second;
if(u==1){
cout<<w;
return 0;
}
pq.pop();
for(auto p: v[u]){
if(w+p.second<dist[p.first]){
pq.push({-w-p.second,p.first});
dist[p.first]=w+p.second;
}
}
}
cout<<"-1";
return 0;
}
