#include <bits/stdc++.h>
using namespace std;
int const INF=1e9;
int const MAX=30005;
int pos[MAX];
int pas[MAX];
int n,m;
void read(){
cin>>n>>m;
int i;
for(i=0;i<m;++i)
cin>>pos[i]>>pas[i];
}
int drum[MAX];
bool ales[MAX];
int dist(int p1,int p2,int pas){
if(p1>p2)
swap(p1,p2);
int dif=p2-p1;
if(dif%pas==0)
return dif/pas;
return INF;
}
void dijkstra(){
int i,j;
for(i=1;i<=m;++i)
drum[i]=INF;
for(j=1;j<m;++j){
int doge=m;
for(i=0;i<m;++i)
if(!ales[i] && drum[i]<drum[doge])
doge=i;
if(doge<m){
ales[doge]=1;
for(i=0;i<m;++i)
if(drum[i]>drum[doge]+dist(pos[doge],pos[i],pas[doge]))
drum[i]=drum[doge]+dist(pos[doge],pos[i],pas[doge]);
}
}
}
void write(){
if(drum[1]>m)
drum[1]=INF;
if(drum[1]==INF)
cout<<-1;
else
cout<<drum[1];
}
int main()
{
read();
dijkstra();
write();
return 0;
}
