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 C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>

int triangle_area(int a)
{
  if(a % 2 == 0) {
    return (a + 1) * a / 2;
  } else {
    return a * (a / 2 + 1);
  }
}

int main()
{
  int n, k;
  scanf("%d %d", &n, &k);
  int cnt = 0;
  for(int a = 1; a <= n; a++) {
    for(int b = 1; b <= a; b++) {
      if(k <= a % b) {
        cnt++;
      }
    }
  }
  cnt += triangle_area(n-k+1) - (n - k+1);
  printf("%d\n", cnt);
  return 0;
}