This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#include "bits/stdc++.h"
using namespace std;
using ll = long long;
const int maxn=3e4+5;
int n,m;
int b[maxn],p[maxn];
vector<int>v[maxn];
map<pair<int,int>,int>dis;
int main(){
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
for(int i = 0;i < m;i++){
cin >> b[i] >> p[i];
v[b[i]].push_back(p[i]);
}
queue<pair<int,int>>q;
q.push({b[0],p[0]});
dis[{b[0],p[0]}] = 0;
while(!q.empty()){
int idx = q.front().first;
int pi = q.front().second;
q.pop();
if(idx == b[1]) return cout<<dis[{idx,pi}],0;
int now = dis[{idx,pi}];
if(idx - pi >= 0){
auto t = dis.find({idx - pi,pi});
if(t == dis.end() or (t != dis.end() and (*t).second > now + 1)){
dis[{idx - pi,pi}] = now + 1;
q.push({idx - pi,pi});
}
}
if(idx + pi < n){
auto t = dis.find({idx + pi,pi});
if(t == dis.end() or (t != dis.end() and (*t).second > now + 1)){
dis[{idx + pi,pi}] = now + 1;
q.push({idx + pi,pi});
}
}
for(auto i : v[idx]){
if(idx - i >= 0){
auto t = dis.find({idx - i,i});
if(t == dis.end() or (t != dis.end() and (*t).second > now + 1)){
dis[{idx - i,i}] = now + 1;
q.push({idx - i,i});
}
}
if(idx + i < n){
auto t = dis.find({idx + i,i});
if(t == dis.end() or (t != dis.end() and (*t).second > now + 1)){
dis[{idx + i,i}] = now + 1;
q.push({idx + i,i});
}
}
}
}
cout<<-1;
}
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