#include <bits/stdc++.h>

#define rep(i, n)   for(int i=0;i<n;i++)
#define mx          1005

using namespace std;

int n, k, c[mx];
int tog[mx];

int main() {
    cin >> n >> k;
    rep(i, n) cin >> c[i];
    int ret = 0;
    rep(i, n){
        if(tog[i] == 0){
            ret += c[i+k];
            for(int j=i;j<=i+2*k;j++) tog[j] = 1;
        }
        
    }
    cout << ret << endl;
    return 0;
}