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 <bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define all(a) (a).begin(), (a).end()
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
const int MOD = 1e9+7;
const ll LINF = 1e18;
const int INF = 1e9;
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;

int main()
{
    cin >> N;
    for (ll i = 2; i*i < N; i++)
    {
        for (ll j = i; j < N; j++)
        {
            if(N-i*j > 0) A += (i==j) ? 1 : 2;
            else break;
        }
        
    }
    A += 2*N-3;
    cout << A;
    
    return 0;
}