#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int n,k;
    cin >> n >>k;
    int ar[n];
    for(int i=0; i<n; i++){
        cin >> ar[i];
    }
    int a=0;
    for(int i=k; i<n-k; ){
        a = a+ar[k];
        i=i+k;
    }
    cout << a;
    return 0;
}