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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
class Program
{
    //static List<string> str = new List<string>();
	static void Main(string[] args)
	{
		// 入力フェーズ
		int N = Int32.Parse(Console.ReadLine());
		string[] input = Console.ReadLine().Split(' ');

		// 並び変えなくても順番に並んでいたらOK
		if(isOrder(N, input))
		{
			Console.WriteLine("YES");
			return;
		}

		// iとj番目を入れ替えて順番に並んでいるかチェックする
		for(int i=0; i<N-1; i++)
		{
				for(int j=i+1; j<N; j++)
				{
					string[] dst = new string[N];
					Array.Copy(input, dst, N);
					if(changeOrder(N, i, j, dst))
					{
						Console.WriteLine("YES");
						return;
					}
				}
		}
		Console.WriteLine("NO");


    }
static bool changeOrder(int N, int i, int j, string[] input)
{
	// iとj番目を入れ替えてチェックする
	string stri = input[i];
	string strj = input[j];
	input[i] = strj;
	input[j] = stri;

	return isOrder(N, input);
}

static bool isOrder(int N, string[] input)
{
	bool ret = true;
	// 順番に並んでいたらOK
	for(int i=0; i<N; i++)
	{
		if(Int32.Parse(input[i]) == (i+1))
			continue;
		else
		{
			ret = false;
			break;
		}
	}
	return ret;
}

}