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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
#define SZ(x) ((int)x.size())
#define ALL(x) (x).begin(), (x).end()
using namespace std;
typedef long long ll;
typedef long double ld;
int n, k;
int main() {
	scanf("%d%d", &n, &k);
	ll res = 0;
	for(int i=k+1; i<=n; i++) {
		ll x = (n - k) / i;
		res += (i - k) * x + min((ll)n, (x + 1) * i - 1) - (i * x + k) + 1;
	}
	printf("%lld", res - (k == 0));
	return 0;
}

// [k, i-1]
// [i+k, 2i-1]
// [2i+k, 3i-1]
// ...
// [xi+k, min(n, (x+1)i - 1)]

// xi+k <= n
// xi <= n - k
// x <= (n - k) / i