#include<bits/stdc++.h>
using namespace std ;
const int maxn = 3e4+123 ;
const int INF=1e9+7 ;
priority_queue <pair<int,int> , vector <pair<int,int>> , greater<pair<int,int>> >bfs ;
vector <int> doge[maxn] ;
int n , m ;
int visited[maxn] ; //exitfrom[maxn][maxn];
void BFS(int t){
while(!bfs.empty()){
int pay=bfs.top().first , ver = bfs.top().second ;
//cout<<ver<<' '<<pay<<' '<<t<<' ' ;
if(ver==t){
cout<<pay ;
return ;
}
bfs.pop() ;
if(visited[ver]==pay){
visited[ver]=pay ;
for(int i:doge[ver]){
//cout<<i<<' ' ;
for(int verp=ver+i ,cnt=1 ; verp<n ; verp += i , cnt++)
if(visited[verp] > cnt+pay){
bfs.push({cnt + pay , verp}) ;
visited[verp]= cnt+pay ;
}
for(int verp=ver-i , cnt=1 ; verp>=0 ; verp-=i , cnt++)
if(visited[verp] > cnt+pay){
bfs.push({cnt + pay , verp}) ;
visited[verp]= cnt+pay ;
}
}
doge[ver].clear() ;
doge[ver].shrink_to_fit();
}
//cout<<bfs.size()<<'\n';
}
cout<<-1 ;
}
int main(){
cin>>n>>m ;
int loc , p;
fill(visited , visited+n+1 , INF) ;
cin>>loc>>p ;
doge[loc].push_back(p) ;
visited[loc]=0 ;
bfs.push({0,loc}) ;
cin>>loc>>p ;
for(int i=2 ; i<m ;i++){
int loco;
cin>>loco>>p ;
doge[loco].push_back(p) ;
}
for(int i=0 ; i<n ; i++){
sort(doge[i].begin() , doge[i].end()) ;
doge[i].erase(unique(doge[i].begin(), doge[i].end()), doge[i].end());
doge[i].shrink_to_fit();
}
BFS(loc) ;
}
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