#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 */ 
    long long int n,k;
    cin>>n>>k;
    
    long long int arr[n];
    for(long long int i=0;i<n;i++)
        {
        cin>>arr[i];
    }
    
    long long bulb=n,s=0,ans=0;
    while(bulb>2*k+1)
        {
        s=s+k;
        ans+=arr[s];
        s=s+k;
        bulb=bulb- (2*k-1);
        
    }
    if(bulb==0)
        cout<<ans<<endl;
    else
        cout<<ans+arr[s];
    
    return 0;
}