Integer Sequences from Addition of Terms

#include<bits/stdc++.h>

using namespace std;

int arr[25];

int main()
{
    int test,i,j,n,d,k,sum,a;

    long long total;

    scanf("%d",&test);

    while(test--)
    {
        scanf("%d",&n);

        for(i=1; i<=n+1; i++)
        {
            scanf("%d",&arr[i]);
        }

        scanf("%d%d",&d,&k);

        total = sum = 0;

        a = d;

        for(i=1 ; ; i++)
        {
            sum+=a;

            if(sum>=k)
                break;

            a = d*(i+1);
        }

        total = arr[1];

        for(j=2;j<=n+1;j++)
        {
            total+= arr[j]*(pow(i,j-1));
        }

        printf("%lld\n",total);
    }

    return 0;
}