#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define F first
#define S second
#define int long long
const int N=3e4+5;
int a[N],b[N],n,m,dis[N];
vector<pair<int,int>> adj[N];
priority_queue<pair<int,int>> pq;
bool vis[N];
void dijkstra(){
pq.push({0,0});
while (!pq.empty()){
pair<int,int> x=pq.top();
x.F*=-1;
pq.pop();
if (vis[x.S]) continue;
vis[x.S]=true;
for (auto u:adj[x.S]){
if (dis[u.F]<=x.F+u.S) continue;
dis[u.F]=x.F+u.S;
pq.push({-dis[u.F],u.F});
}
}return;
}
int32_t main(){
cin>>n>>m;
for (int i=1;i<N;i++) dis[i]=INT_MAX;
for (int i=0;i<m;i++) cin>>a[i]>>b[i];
for (int i=0;i<m;i++){
for (int j=0;j<m;j++){
if (i==j) continue;
if (abs(a[i]-a[j])%b[i]) continue;
adj[i].pb({j,abs(a[i]-a[j])/b[i]});
//cout<<i<<' '<<j<<' '<<abs(a[i]-a[j])/b[i]<<endl;
}
}
dijkstra();
if (dis[1]==INT_MAX) cout<<-1<<endl;
else cout<<dis[1]<<endl;
return 0;
}
