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 whiteread::parse_line;

fn main() {
    let n: usize = parse_line().unwrap();
    let mut t: usize = 0;
    let mut a: usize = 1;
    while a <= n {
        let mut b: usize = 1;
        while b <= n / a {
            if a*b < n {
                t = t + 1;
            }
            b = b + 1;
        }
        a = a + 1;
    }
    println!("{}", t);
}