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


int main() {
    int n,m,i=0,c=0;
    cin>>n>>m;
    int arr[n];
    for(i=0;i<n;i++)
        cin>>arr[i];
    for(i=0;i<n;i=i+((m*2)+1))
        {
        c=c+arr[i];
    }
    cout<<c;
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    return 0;
}