#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define inf 1e18+100
using namespace std;
const ll N=30300;
priority_queue<pair<ll,ll>>q;
ll n,m;
ll b[N],p[N],dist[N];
vector<ll>a[N];
vector<pair<ll,ll>>v[N];
void fil(ll id){
for(ll i=0;i<n;i++){
if(i==id||a[i].empty())continue;
for(auto j:a[id]){
if((i%p[j])==(id%p[j])){
v[id].pb({i,abs(i-id)/p[j]});
}
}
}
}
int main(){
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin>>n>>m;
for(ll i=0;i<m;i++){
cin>>b[i]>>p[i];
a[b[i]].pb(i);
}
for(ll i=0;i<n;i++){
if(!a[i].empty())fil(i);
}
/*for(ll i=0;i<n;i++){
if(v[i].empty())continue;
cout<<i<<": ";
for(auto j:v[i])cout<<'('<<j.first<<' '<<j.second<<')'<<" ";cout<<endl;
}*/
for(ll i=0;i<n;i++){
dist[i]=inf;
}
q.push({0,b[0]});
dist[b[0]]=0;
while(!q.empty()){
ll u=q.top().second,d=-q.top().first;
q.pop();
if(d>dist[u])continue;
for(auto i:v[u]){
ll ch=i.first,chd=i.second;
if(chd+d<dist[ch]){
dist[ch]=chd+d;
q.push({-dist[ch],ch});
}
}
}
if(dist[b[1]]==inf){cout<<-1<<endl;return 0;}
cout<<dist[b[1]]<<endl;
return 0;
}
