#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
int main() {
     LL n,k,i,j,ans = 0;
     cin >> n >> k;
      LL a[n+1];
      LL c[n+1];
      for(i = 1;i <= n;i++)
          cin >> c[i];
      ans = 0;
     i = k+1;
    //  flag = 0;
    while(i <=n)
    {
       ans += c[i];
       j = i + k;
       i += (2*k+1);
     }
   if(j < n)
      ans + (c[(n-j + 1)/2]);
       cout << ans << endl;
       
    return 0;
}