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 JavaScript (node.js v5.12) (judge verdict: Accepted)

'use strict'

const main = input => {
	const args = input.split('\n').map(arg => arg.split(' '))
	const n = parseInt(args[0][0], 10)
	const p = args[1].map(arg => parseInt(arg, 10))
	let flg = false

	let t2 = true
	for (let k = 0; k < n; k++) {
		if ((k + 1) !== p[k]) t2 = false
	}
	if (t2) flg = true
	
	for (let i = 0; i < n; i++) {
		for (let j = (i + 1); j < n; j++) {
			const t1 = p.slice()
			t1[i] = p[j]
			t1[j] = p[i]
			let t2 = true
			for (let k = 0; k < n; k++) {
				if ((k + 1) !== t1[k]) t2 = false
			}
			if (t2) flg = true
		}
	}

	if (flg) console.log('YES')
	else console.log('NO')
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'))