#include<stdio.h>
typedef struct TreeNode(){
    int data;
}Node;
int main(){
    int N,L;
    scanf("%d",&N);
    scanf("%d",&L);
    Node arr[i];
    for(int i=0;i<N;i++){
        scanf("%d",&arr[i].data)7;
    }
    int sum=0;
    int node=0;
    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++;
            node+=2;
            count=0;
        }
    }
    printf("%d",sum);
}