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(unused, non_snake_case, dead_code, non_upper_case_globals)]
use {
    proconio::{marker::*, *},
    std::*,
    std::{cmp::*, collections::*, convert::*, iter::*, marker::*, mem::*, ops::*},
};
fn main() {
    input! {//
        n:usize,
    }
    let mut ans = 0i64;
    for a in 1..=n {
        for b in 1..=((n + a - a) / a) {
            if a * b < n {
                ans += 1;
            }
        }
    }
    println!("{}", ans);
}