#include<stdio.h>
typedef struct node{
    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]);
    
    int count=0;
    int node=1;
    int level=0;
    int sum=0;
    count++;
    if(level==1){
        sum+=arr[i];
    }
    if(count==node){
        count=0;
        level++;
        node**=2;
    }
    }
    printf("%d",sum);
    return 0;
}