#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main() {
    long int i,n,cou=0;
    int l=0,k;
    scanf("%ld %d",&n,&k);
    long int a[n];
    for(i=0;i<n;i++)
        scanf("%ld",&a[i]);
    for(i=0;i<n;i++){
        if(l++==k){
            cou=cou+a[i];
            l=0;
        }
        
    }
    printf("%ld",cou);
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    return 0;
}