#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);
    }
    if(!scanf("%d",&L) || i<0 || L>N){
        printf("Invalid input");
        return 0;
    }
    
    Node arr[N];
    for(int i=0;i<N;i++){
        if(!scanf("%d",&arr[i].data)){
            printf("Invalid input");
            return 0;
        }
    }
    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++;
        node+=2;
        count=0;
    }
}
printf("%d",sum);
}