#include<bits/stdc++.h>
using namespace std;
map<pair<short,short>,vector<short>>mp;
int up1[2000005];
int up2[2000005];
int down1[2000005];
int down2[2000005];
int hor[2000005];
int dis[2000005];
int inf=1e9;
int have[30005];
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n,m;cin>>n>>m;
int st=0,en=0;
for(int i=0;i<m;i++){
int b,p;cin>>b>>p;
if(i==0)st=b;
if(i==1)en=b;
if(i!=1)mp[{b%p,p}].push_back(b);
have[b]++;
}
for(int i=0;i<2e6;i++)up1[i]=up2[i]=down1[i]=down2[i]=hor[i]=-1;
int id=n-1;
for(auto x:mp){
int cur=x.first.first;
int p=x.first.second;
for(auto b:x.second)up1[b]=id+(b-cur)/p+1;
for(cur;cur<n;cur+=p){
id++;
if(have[cur])down1[id]=cur;
if(cur!=x.first.first)hor[id-1]=id;
}
cur=x.first.first;
for(auto b:x.second)up2[b]=id+(b-cur)/p+1;
for(cur;cur<n;cur+=p){
id++;
if(have[cur])down2[id]=cur;
if(cur!=x.first.first)hor[id]=id-1;
}
}
mp.clear();
for(int i=0;i<=id;i++)dis[i]=inf;
deque<pair<int,int>>dq;
dq.push_back({0,st});
while(!dq.empty()){
auto [d,u]=dq.front();
//cerr<<u<<" "<<d<<"\n";
dq.pop_front();
if(d>=dis[u])continue;
dis[u]=d;
if(u==en)break;
if(up1[u]!=-1)dq.push_front({d,up1[u]});
if(up2[u]!=-1)dq.push_front({d,up2[u]});
if(down1[u]!=-1)dq.push_front({d,down1[u]});
if(down2[u]!=-1)dq.push_front({d,down2[u]});
if(hor[u]!=-1)dq.push_back({d+1,hor[u]});
}
if(dis[en]==inf)cout<<-1;
else cout<<dis[en];
}
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