I am getting WA despite reasonable logic

#1

So using sieve I calculate the factors of each number, then I calculated the sum of all the factors for all elements, stored then in a hash table.
And answered all the queries using a single look up.
Here is my code:

#include <bits/stdc++.h>
using namespace std;
#define MX 10000
#define ll long long
#define endl "\n"
vector<int> factorSumIndex(MX+1,-1);

ll Pow(ll a, int b){
    ll ans = 1;
    while(b>0){
        if(b&1){
            ans = ans *a;
        }
        a = a*a;
        b = b>>1;
    }
    return ans;
}

void init(){
    vector<vector<pair<int,int> > > factors(MX+1);
    vector<int> sieve(MX+1);
    for(int i=1;i<=MX;i++){
        sieve[i] = i;
    }
    for(int i=2;i<=MX;i++){
        if(sieve[i] == i){
            factors[i].push_back(make_pair(i,1));
            for(int j = i+i;j<=MX;j+=i){
                int cnt = 0;
                while(sieve[j]%i == 0){
                    sieve[j]/=i; cnt++;
                }
                factors[j].push_back(make_pair(i,cnt));
            }
        }
    }
    for(int i=1;i<=MX;i++){
        ll sum = 1;
        vector<pair<int,int> > &factor = factors[i];
        for(int j=0;j<factor.size();j++){
            sum = sum * ( (Pow(factor[j].first,factor[j].second+1) - 1) / (factor[j].first - 1) );
        }
        if(sum<=MX){
            factorSumIndex[sum] = i;
        }
    }
}

int main(){
    init();
    int num;
    cin>>num;
    while(num!=0){
        cout<<factorSumIndex[num]<<endl;
        cin>>num;
    }
    return 0;
}