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 <iostream>
#include <map>
#include <vector>
#include <algorithm>
#include <queue>
#include <math.h>
#include <stack>
#include <set>
#include <deque>

using namespace std;
typedef long long ll;
ll mod = 1e9+7;
#define rep(i,n) for(int i = 0; i < (n); ++i)

int main(){
    ll n, k;
    cin >> n >> k;
    ll ans = 0;
    if ( k == 0 ) {
        cout << n * n << endl;
        return 0;
    }
    for ( ll i = k + 1; i <= n; ++i ) {
        int mod_group_num = n / i;
        int cntup_num = i - k;
        int rest_num = n - mod_group_num * i;
        int add_cntup_num = max(0LL, rest_num - k + 1);
        ans += cntup_num * mod_group_num + add_cntup_num;
    }
    cout << ans << endl;
    return 0;
}