#include <stdio.h>
typedef struct TreeNode{
    int data;
}Node;
int main(){
    int N,L;
    scanf("%d",&N);
    scanf("%d", &L);
    Node arr[N];
    for(int i=0;i<N;i++){
        scanf("%d", &arr[i].data);
    }
    int sum=0;
    int node=1;
    int level=0;
    int count=0;
    for(int i=0;i<N;i++){
        if(level==L){
            sum+=arr[i].data;
        }
        count++;
        if(count == node)
          level++;
          count = 0;
    }
}
printf("%d",sum);
}