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)

use proconio::{input, fastout};
use proconio::marker::{Chars};
use proconio::marker::Usize1;
use std::cmp::*;

macro_rules! debug {
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}

// fn num_divisor(x: usize) -> usize {
//     let s = (x as f64).sqrt() as usize;
//     let mut cnt = 0;
//     for i in 1..s+1 {
//         if x%i==0 {
//             cnt += 1;
//         }
//     }
//     if s*s==x {
//         cnt*2-1
//     } else {
//         cnt*2
//     }
// }

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

    let mut ans = 0;
    for i in 1..n {
        ans += (n-1)/i;
        // debug!(ans);
    }

    println!("{}", ans);

}