Problem:
Score : 400 points

Problem Statement

For an integer N, we will choose a permutation \{P_1, P_2, ..., P_N\} of \{1, 2, ..., N\}.

Then, for each i=1,2,...,N, let M_i be the remainder when i is divided by P_i.

Find the maximum possible value of M_1 + M_2 + \cdots + M_N.

Constraints

N is an integer satisfying 1 \leq N \leq 10^9.

Input

Input is given from Standard Input in the following format:

N

Output

Print the maximum possible value of M_1 + M_2 + \cdots + M_N.

Sample Input 1

2

Sample Output 1

1

When the permutation \{P_1, P_2\} = \{2, 1\} is chosen, M_1 + M_2 = 1 + 0 = 1.

Sample Input 2

13

Sample Output 2

78

Sample Input 3

1

Sample Output 3

0

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::io::prelude::*;

fn input<T: std::str::FromStr>() -> T {
    let stdin = std::io::stdin();
    let it = stdin.lock().bytes().map(|c| c.unwrap() as char);
    let (fs, ft) = (|c: &char| c.is_whitespace(), |c: &char| !c.is_whitespace());
    let token = it.skip_while(fs).take_while(ft).collect::<String>();
    token.parse().ok().unwrap()
}
#[allow(dead_code)]
fn with_stdout_lock<F: FnOnce(std::io::BufWriter<std::io::StdoutLock>)>(f: F) {
    let out = std::io::stdout();
    f(std::io::BufWriter::new(out.lock()));
}
#[allow(unused_macros)]
macro_rules! debug {
    ($($var:expr),*) => { #[cfg(debug_assertions)] {
        $({ match &$var { v => eprint!("{}={:?} ", stringify!($var), v) } })*
        eprintln!();
    }};
}

fn main() {
    let n: i64 = input();
    println!("{}", (0..n).sum::<i64>());
}