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>
using namespace std;
using ll=long long;
int main(){
    ll n;
    int k;
    cin>>n>>k;
    if(k==0){
        cout<<n*n<<endl;
        return 0;
    }
    ll res=0;
    for(int b=k+1;b<=n;b++){
        ll shuki=(n+1)/b;
        ll par=b-k;
        ll rest=n%b-k+1;
        if(rest<0 || (n+1)%b==0) rest=0;
        res+=par*shuki+rest;
    }
    cout<<res<<endl;
    
    return 0;
}