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 Rust (1.42.0) (judge verdict: Accepted)

#[allow(unused_imports)] use proconio::{input, marker::{Bytes, Chars, Usize1, Isize1}};
#[allow(unused_imports)] use std::cmp::{min, max};
#[allow(unused_imports)] use superslice::Ext as _;

#[proconio::fastout]
fn main() {
	input! {
		n: usize,
	}

	let mut ans = 0usize;
	let r = (n as f64).sqrt() as usize + 1;
	for a in 1..=r {
		for b in a..n {
			let ab = a * b;
			if ab >= n {
				break;
			}
			ans += 1;
			if a != b {
				ans += 1;
			}
			//println!("{}, {}, {}", a, b, n-ab);
		}
	}

	println!("{}", ans);
}