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)

//Md. Shajibul Islam..
//East West University, CSE Department'19

#include<bits/stdc++.h>
using namespace std;
#define FAST             ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define loop(i,n)        for(int i=0;i<n;i++)
#define CASE_PRINT       cout<<"Case "<<C<<": "
#define CASE_PRINT2      cout<<"Case "<<C<<":"<<endl
#define ll               long long
#define ld               long double
#define Pi               2*acos(0.0) // acos(-1.0)

int main()
{
    FAST;
    ll n;cin>>n;
    int ans = n-1;
    for(int i=2;i<n;i++)
    {
        ans = ans + (n/i);
        if(n%i==0)ans--;
    }

    cout<<ans<<endl;
}