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


int main() {
    int n,k;
        cin>>n>>k;
    int a[n]; 
    int l;
    l=n-1-k;
    for(int i=0;i<n;i++)
      {
        cin>>a[i];
    }
  
    int sum =0;
    int c[1000];
    while(1)
        {
        sum =a[l] + sum;
        l=l-k;
      
        if(l<=0)break;
    }
    cout<<sum;
    return 0;
}