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)

#![allow(dead_code)]
use std::io::*;

fn main(){
    let n: usize = read();
    let a: Vec<usize> = (0..n).map(|_| read()).collect();
    let mut cnt = 0;
    for i in 1..n + 1{
        if a[i - 1] != i {
            cnt += 1;
        }
    }
    let ans = if cnt == 0 || cnt == 2 {"YES"} else {"NO"};
    println!("{}", ans);
}

fn upper_bound(a: &Vec<i32>, x: &i32) -> Option<usize>
{
    let mut ok: isize = a.len() as isize;
    let mut ng: isize = -1;
    if ok == 0 || a[ok as usize - 1] <= *x{
        None
    }else{
        while (ng - ok).abs() > 1{
            let mid = (ok + ng) / 2;
            if a[mid as usize] > *x{
                ok = mid;
            }else{
                ng = mid;
            }
        }
        Some(ok as usize)
    }
}

fn read<T>() -> T
where
    T: std::str::FromStr,
{
    let stdin = std::io::stdin();
    let token: String = stdin
        .lock()
        .bytes()
        .map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().unwrap()
}