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)

#include <stdio.h>
#include <math.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int main(){
	long long int n, k, i, j, p, q, ans=0;
	scanf("%lld%lld", &n, &k);
	// k=0
	if(k==0){
		printf("%lld\n", n*n);
	}
	else{
		// k>0
		// a<b
		for(i=n; i>k; i--){
			ans+=i-k;
		}
		// printf("%lld\n", ans);
		// a>b
		for(i=n-k; i>k; i--){
			p=n%i;
			q=n/i;
			if(p>=k && q==0){
				ans+=p-k+1;
			}
			else if(p>=k){
				ans+=(i-k)*(q-1)+p-k+1;
			}
			else{
				ans+=(i-k)*(q-1);
			}
		}
		printf("%lld\n", ans);
	}
	return 0;
}