#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,node=1,level=0,count=0;
    for(int i=0;i<n;i++){
        if(level==1){
            sum+=arr[i].data;
        }
        count++;
        if(count==node){
            level++;
            node*=2;
            count=0
        }
    }
    printf("%d",sum);
}