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: Accepted)

/*
 AtCoder XXX 000
 */
 
#include <stdio.h>
 
int main(void){
  unsigned long long n,k;
  unsigned long long i;
  unsigned long long cnt;
  
  scanf("%llu %llu",&n,&k);
  
  cnt=0;
  if(k==0)
  {
    printf("%llu\n",n*n);
    return 0;
  }
  
  for(i=1; i<=n; i++){
    if(i>k)
    {
      cnt += (i-k)*(n-n%i)/i;
      cnt += (n%i)>=k ? (n%i)-k+1 : 0;
    }
  }
  
  printf("%llu\n",cnt);
  return 0;
}