#include<stdio.h>
int main()
{
    int n,L;
    
    scanf("%d",&n);
     scanf("%d",&L);
     if(n<=0 || L<0)
     {
          printf("Invalid input");
          return 0;
     }
     int arr[n];
     for(int i=0;i<n;i++)
     {
         if((scanf("%d",&arr[i]))
         {
            printf("Invalid input");
          return 0;
         }
     }
     int s =0;
     int in = 0;
      for(int level = 0;level <= L && in < n; level++)
     {
         int count = level + 1;
          for(int i = 0; i < count && in < n; i++)
     {
         s += arr[in++];
     }
     }
     printf("%d",sum);
     return 0;
}