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)

#![allow(non_snake_case)]
#![allow(unused_imports)]

use proconio::input;
use std::cmp::{max, min};
use std::collections::{HashMap, HashSet};

fn main() {
    input! {
        N: usize,
    }
    let mut ans = 0;
    for a in 1..N {
        for b in a..N {
            let c = (N as isize) - ((a * b) as isize);
            if c > 0 {
                if a == b {
                    ans += 1;
                } else {
                    ans += 2;
                }
            } else {
                break;
            }
        }
    }
    println!("{}", ans);
}