#include<bits/stdc++.h>
#define int long long
#define ss second
#define ff first
#define pb push_back
const int mxn=30005;
using namespace std;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n,m;
cin>>n>>m;
int b[m],p[m];
vector<int>a[n];
for(int i=0;i<m;i++){
cin>>b[i]>>p[i];
a[b[i]].pb(p[i]);
}
set<pair<int,int>>s;
s.insert({0,b[0]});
vector<int>d(n,1e9);
d[b[0]]=0;
while(!s.empty()) {
pair<int,int> q=*s.begin();
s.erase(s.begin());
int C=q.ff,p=q.ss;
for(int i:a[p]) {
int c=p,t=0;
while(c>=0){
if(C+t<d[c]) {
if(d[c]<1e9) s.erase({d[c],c});
d[c]=C+t;
s.insert({d[c],c});
}
c-=i;
t++;
}
c=p;
t=0;
while(c<n) {
if(C+t<d[c]) {
if(d[c]<1e9) s.erase({d[c],c});
d[c]=C+t;
s.insert({d[c],c});
}
c+=i;
t++;
}
}
}
if(d[b[1]]==1e9){
cout<<"-1";
} else {
cout<<d[b[1]];
}
}
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