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_imports)]

//input
use proconio::input;
use proconio::marker::{Bytes, Chars};
//use std::io::Read;

//collections
//use std::collections::BTreeMap;
//use std::collections::BTreeSet;
//use std::collections::HashMap;
//use std::collections::HashSet;
//use std::collections::VecDeque;
//use std::collections::BinaryHeap;

//use std::cmp::*;

fn main() {
    input! {
        n: i64,
    }
    if n == 2 {
        println!("1");
        return;
    }
    let mut ans = 0;
    for i in 1..n - 1 {
        if i * i >= n {
            break;
        }
        let mut ok = i;
        let mut ng = n;
        while (ok - ng).abs() > 1 {
            let mid = (ok + ng) / 2;
            if i * mid < n {
                ok = mid;
            } else {
                ng = mid;
            }
        }
        ans += (ok - i) * 2 + 1;
        //println!("{}", ok - i);
    }
    println!("{}", ans);
}