// editor2
#include<stdio.h>
#include<stdlib.h>
typedef struct node{
    char data;
}nd;
int main(){
    int n,l;
    scanf("%d",&n);
    if(n<0){
        printf("Invalid input");
        return 0;
    }
    scanf("%d",&l);
    nd *a[100];
    for(int i=0;i<n;i++)
    {
        scanf("%d",&a[i]);
    }
    int count=0;
    int level=0;
    int node=1;
    int sum=0;
    int reached=0;
    for(int i=0;i<n;i++)
    {
    if(level==l)
    {
        sum+=a[i];
        reached=1;
    }
    count++;
    if(count==node)
    {
        level++;
        node*=2;
        count=0;
    }
    if(!reached){
        printf("Invalid input");
    }
    else
    {
    printf("%d",sum);
    }
    return 0;
    
}