#include<stdio.h>
int main()
{
    int n,L;
    cin>>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(!(cin >> arr[i]))
         {
           cout << "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++];
     }
     }
    cout << s;
     return 0;
}