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};

#[fastout]
fn main() {
    input! {
        n: u64,
    };

    let mut ans = 0;
    for a in 1..=n {
        for b in 1..=n {
            if a*b < n {
                ans += 1;
            } else {
                break;
            }
        }
    }

//    for i in 1..n {
//        if i*i < n {
//            ans += n - i*i;
//        } else {
//            break;
//        }
//    }

    println!("{}", ans);
}