Problem:
Score : 200 points

Problem Statement

We have a sequence p = {p_1,\ p_2,\ ...,\ p_N} which is a permutation of {1,\ 2,\ ...,\ N}.

You can perform the following operation at most once: choose integers i and j (1 \leq i < j \leq N), and swap p_i and p_j. Note that you can also choose not to perform it.

Print YES if you can sort p in ascending order in this way, and NO otherwise.

Constraints

All values in input are integers.

2 \leq N \leq 50

p is a permutation of {1,\ 2,\ ...,\ N}.

Input

Input is given from Standard Input in the following format:

N
p_1 p_2 ... p_N

Output

Print YES if you can sort p in ascending order in the way stated in the problem statement, and NO otherwise.

Sample Input 1

5
5 2 3 4 1

Sample Output 1

YES

You can sort p in ascending order by swapping p_1 and p_5.

Sample Input 2

5
2 4 3 5 1

Sample Output 2

NO

In this case, swapping any two elements does not sort p in ascending order.

Sample Input 3

7
1 2 3 4 5 6 7

Sample Output 3

YES

p is already sorted in ascending order, so no operation is needed.

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

use std::io::*;
use std::str::FromStr;

fn parse_line<T: FromStr>() -> T {
    let mut s = String::new();
    stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn parse_line_vec<T: FromStr>() -> Vec<T> {
    let line = parse_line::<String>();
    line.split_whitespace()
        .map(|s| s.parse().ok().unwrap())
        .collect()
}

fn println<T: ToString>(s: T) {
    println!("{}", s.to_string());
}

// ------------------------------------------------------------------------

fn is_sorted(p: &Vec<u32>) -> bool {
    for p_sub in p.windows(2) {
        if p_sub[0] >= p_sub[1] {
            return false;
        }
    }
    true
}

fn main() {
    let n = parse_line::<usize>();

    let p = parse_line_vec::<u32>();

    if is_sorted(&p) {
        println("YES");
        return;
    }

    for i in 0..n - 1 {
        for k in i + 1..n {
            let mut p2 = p.clone();
            p2.swap(i, k);
            if is_sorted(&p2) {
                println("YES");
                return;
            }
        }
    }
    
    println("NO");
}