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 itertools::Itertools;
use proconio::{input, marker::*};

fn main() {
    input! {
        n: usize
    };
    if n == 2 {
        println!("1");
        return;
    }
    // c>0 の範囲でaとbは自由なので, n-1以下になる整数の組み合わせ総数が解
    let mut count = 0;
    for a in 1..=(n - 1) {
        count += (n - 1) / a;
    }
    println!("{}", count);
}