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,
  unused_variables,
  unused_assignments,
  unused_mut,
  unused_imports,
  dead_code
)]
use proconio::{input, marker::*};
use std::cmp::*;
use std::collections::*;
//use std::num;
//use petgraph::unionfind::UnionFind;
//use permutohedron::LexicalPermutation as _;

const MOD: i64 = 1000_000_007;
//#[fastout()]
fn main() {
  input! {
      N:usize,
  }
  let mut ans = 0;
  for a in 1..N {
    let b = (N-1)/a;
    ans += b;
  }
  println!("{}", ans);
}