WA求助

#include<iostream>
#define int long long
using namespace std;
int a,b,s=1,ans;
const int mod=9901;
signed main(){
    cin>>a>>b;
    if(a==1){
        cout<<1;
        return 0;
    }
    if(a==0){
        cout<<0;
        return 0;
    }
    for(register int i=1;i<=b+1;++i){
        ans=(ans+s)%mod;
        s=(s*a)%mod;
    }
    cout<<ans;
    return 0;
}