#include <bits/stdc++.h>
//#include "game.h"
using namespace std;
vector<long long> ind[30001];
long long len[30001];
long long n,m;
long long nah,z;
long long d(){
long long cost[n+1];
for(int i = 0;i<=n;i++)cost[i] = 1e18;
priority_queue<pair<long long,long long>> q1;
q1.push({0,z});
cost[z] = 0;
while(!q1.empty()){
long long co = q1.top().first*-1;
long long no = q1.top().second;
//cout<<lin<<" "<<indx<<"\n";
q1.pop();
if(co>cost[no]) continue ;
for(auto i:ind[no]){
long long c = 1;
for(long long j = no+len[i];j<n;j+=len[i]){
if(cost[j]>cost[no]+c){
cost[j] = cost[no]+c;
q1.push({cost[j]*-1,j});
}
c++;
}
c = 1;
for(long long j = no-len[i];j>=0;j-=len[i]){
if(cost[j]>cost[no]+c){
cost[j] = cost[no]+c;
q1.push({cost[j]*-1,j});
}
c++;
}
}
}
return cost[nah];
}
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
cin>>n>>m;
for(int i = 0;i<m;i++){
long long x,b;
cin>>x>>b;
if(i==1)nah = x;
if(i==0)z = x;
len[i] = b;
ind[x].push_back(i);
}
//cout<<z<<" "<<nah<<"\n";
long long val = d();
if(val==1e18)cout<<-1<<"\n";
else cout<<val<<"\n";
}
