Problem:
Score : 400 points

Problem Statement

Takahashi had a pair of two positive integers not exceeding N, (a,b), which he has forgotten.
He remembers that the remainder of a divided by b was greater than or equal to K.
Find the number of possible pairs that he may have had.

Constraints

1 \leq N \leq 10^5

0 \leq K \leq N-1

All input values are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the number of possible pairs that he may have had.

Sample Input 1

5 2

Sample Output 1

7

There are seven possible pairs: (2,3),(5,3),(2,4),(3,4),(2,5),(3,5) and (4,5).

Sample Input 2

10 0

Sample Output 2

100

Sample Input 3

31415 9265

Sample Output 3

287927211

Solution in Python (3.4.3) (judge verdict: Accepted)

ans = 0
N,K = map(int,input().split())
for b in range(K+1,N+1):
    x1,y1 = divmod(N-K,b)
    x2,y2 = divmod(N-b+1,b)
    if x1==x2:
        ans += (x1+1)*(y1-y2+1)
    else:
        ans += (x1+1)*(y1+1) + (x2+1)*(b-y2)+b*(x1+x2+2)*(x1-x2-1)//2
if K == 0:
    print(N*N)
else:
    print(ans)