This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include<bits/stdc++.h>
using namespace std;
const int maxn=30000+10;
int n,m,vis[maxn],dis[maxn],av;
set<int>st[maxn],fst[maxn];
vector<int>adj[maxn];
void upd(int ind,int p){
if(st[ind].count(p)!=0){
return ;
}
st[ind].insert(p);
fst[ind].insert(p);
for(int i=ind-p;i>=0;i-=p){
adj[i].push_back(i+p);
adj[i+p].push_back(i);
fst[i].insert(p);
}
for(int i=ind+p;i<n;i+=p){
adj[i].push_back(i-p);
adj[i-p].push_back(i);
fst[i].insert(p);
}
}
void solve(){
set<pair<int,pair<int,int>>>di;
for(auto x:st[av]){
di.insert(make_pair(0,make_pair(av,x)));
}
while((int)di.size()!=0){
auto x=*di.begin();
di.erase(x);
if(x.second.first<0||x.second.first>=n){
continue;
}
if(fst[x.second.first].count(x.second.second)==0){
continue;
}
if(vis[x.second.first]==0){
vis[x.second.first]=1;
dis[x.second.first]=x.first;
}
di.insert(make_pair(x.first+1,make_pair(x.second.first-x.second.second,x.second.second)));
di.insert(make_pair(x.first+1,make_pair(x.second.first+x.second.second,x.second.second)));
fst[x.second.first].erase(x.second.second);
for(auto y:st[x.second.first]){
di.insert(make_pair(x.first+1,make_pair(x.second.first-y,y)));
di.insert(make_pair(x.first+1,make_pair(x.second.first+y,y)));
}
st[x.second.first].clear();
}
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin>>n>>m;
int s=0;
for(int i=0;i<m;i++){
int u,v;
cin>>u>>v;
upd(u,v);
if(i==1){
s=u;
}
if(i==0){
av=u;
}
}
solve();
if(vis[s]){
cout<<dis[s]<<"\n";
}
else{
cout<<-1<<"\n";
}
}
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