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: Accepted)

#include<bits/stdc++.h>
#define ll long long
using namespace std;
int n, k;
ll ans;
int main()
{
    //freopen("in.txt", "r", stdin);
    //freopen("out.txt","w",stdout);
    scanf("%d%d", &n, &k);
    if(k == 0) printf("%lld\n", 1ll * n * n);
    else
    {
        for(int b = k + 1; b <= n; b ++)
        {
            int x = n / b;//5 / 3 = 1

            ans += 1ll * x * (b - k) + max(0ll, 1ll * (n % b - k + 1));
        }
        printf("%lld\n", ans);
    }
    return 0;
}