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; 
long long n,m;
long long a[2009];
long long p[2009];
long long done[2009][2009];
long long dp[2009][2009];
vector<long long>v[10009];
long long inf=1000000000000009ll;
long long bt(long long id,long long dog,long long yes){
        if(yes==1)return inf;
        done[id][dog]=1;
        if(id<0 || id>n)return inf;
        if(id==a[1])return 0;
        long long &ret=dp[id][dog];
        if(ret!=-1)return ret;
        ret=inf;
        if(id+p[dog]<=n && id-p[dog]>0)ret=min(bt(id+p[dog],dog,done[id+p[dog]][dog]),bt(id-p[dog],dog,done[id-p[dog]][dog]))+1;
        else if(id+p[dog]<=n) ret=min(ret,bt(id+p[dog],dog,done[id+p[dog]][dog]))+1;
        else if(id-p[dog]>0) ret=min(ret,bt(id-p[dog],dog,done[id-p[dog]][dog]))+1;
        for(int i=0;i<v[id].size();i++){
                long long D=v[id][i];
                ret=min(ret,bt(id,D,done[id][D]));
        }
        return ret;
}
int main(){
        memset(dp,-1,sizeof(dp));
        cin>>n>>m;
        for(int i=0;i<m;i++){
                cin>>a[i]>>p[i];
                v[a[i]].push_back(i);
        }
        long long x=bt(a[0],0,0);
        if(x!=inf) cout<<x<<endl;
        else cout<<-1<<endl;
}
Compilation message (stderr)
skyscraper.cpp: In function 'long long int bt(long long int, long long int, long long int)':
skyscraper.cpp:21:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=0;i<v[id].size();i++){
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