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 ld long double
#define pb push_back
#define all(x) begin(x), end(x)
#define SZ(x) (int)(x).size()
#define cps(x) sort(all(x)), (x).erase(unique(all(x)), end(x))
#define cps2(x, y) sort(all(x), y), (x).erase(unique(all(x)), end(x))
#define mem(x, y) memset(& x, y, sizeof(x))
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

int a[103], b[103];

int main() {
	ios_base::sync_with_stdio(false), cin.tie(NULL);
	int n; cin >> n;
	ll ans =0 ;
	for (int i = 1; i <= n; i++)
		ans += (n - i + (i - 1)) / i;
	cout << ans;
}