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: Time Limit Exceeded)

use std::io::*;
use std::str::FromStr;

//https://qiita.com/tubo28/items/e6076e9040da57368845
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

fn enum_divisors(n: usize) -> usize{
    let mut res: Vec<usize> = Vec::new();
    let mut i = 1;
    while i*i <= n{
        if n % i == 0 {
            res.push(i);
            if n/i != i {
                res.push(n/i);
            }
        }
        i += 1;
    }
    res.sort();
    res.len()
}

fn main() {
    let n:usize = read();
    let mut ans = 0;
    for i in 1..=(n-1){
        ans += enum_divisors(n-i);
    }
    println!("{}",ans);
}