Ryotsu loves Primes

Ryotsu loves playing with prime numbers. His father was a mathematics professor. To test Ryotsu's knowledge of primes, his father gave him a simple task. The task is to find the sum of every Kth prime number in the given list of numbers. He's having trouble solving it, being an elementary school student. Help him accomplish the task.

Example:

Input:  n = 7, k = 2, a = {1, 2, 13, 3, 5, 6, 7}
Output: 18

Approach

C++

#include <bits/stdc++.h>
using namespace std;

bool dp[1000001];

void primeInitialize()
{
    memset(dp, true, sizeof(dp));
    dp[0] = false;
    dp[1] = false;
    for (long long i = 2; i * i <= 1000001; i++)
    {
        if (dp[i])
        {
            for (long long j = i * i; j <= 1000001; j += i)
                dp[j] = false;
        }
    }
}
long long sumOfEveryKthPrime(long long n,
                             long long k,
                             long long a[])
{

    long long cnt = 0;
    long long res = 0;
    for (long long i = 0; i < n; i++)
    {
        if (dp[a[i]])
            cnt++;
        if (cnt == k)
        {
            cnt = 0;
            res += a[i];
        }
    }
    return res;
}
int main()
{

    primeInitialize();
    long long n = 7, k = 2;

    long long a[n] = {1, 2, 13, 3, 5, 6, 7};

    cout << sumOfEveryKthPrime(n, k, a) << "\n";

    return 0;
}