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 proconio::input;

fn divisors(n: usize) -> usize {
    let mut c = 0;
    for i in 1.. {
        if i * i > n {
            break;
        }
        if n % i == 0 {
            c += 1;
            if i != n / i {
                c += 1;
            }
        }
    }
    c
}

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