Problem:
Score : 300 points

Problem Statement

Given is a positive integer N.
How many tuples (A,B,C) of positive integers satisfy A \times B + C = N?

Constraints

2 \leq N \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

3

Sample Output 1

3

There are 3 tuples of integers that satisfy A \times B + C = 3: (A, B, C) = (1, 1, 2), (1, 2, 1), (2, 1, 1).

Sample Input 2

100

Sample Output 2

473

Sample Input 3

1000000

Sample Output 3

13969985

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <iostream>
#include<bits/stdc++.h>
using namespace std;

int main()
{
   long long int i,ans=0,n,count=0;
   cin>>n;
  for(i=1;i*i<=n;i++){
    if(n%i==0){
            ans+=(n/i-1)*2;
    }
    else{
            ans+=(n/i)*2;
    }
    count+=i*2-1;
  }
  if((i-1)*(i-1)==n)count--;
  printf("%lld\n",ans-count);
    return 0;
}