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;

class Program
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());

        string[] input = Console.ReadLine().Split(' ');
        int[] b = new int[n];
        int[] c = new int[n];
        int i = 0;
        foreach (string s in input)
        {
            b[i] = int.Parse(s);
            c[i] = b[i];
            i++;
        }
        for (int ii = 0; ii < n - 1; ii++)
        {
            for (int jj = ii+1; jj < n; jj++)
            {
                if (b[ii] > b[jj])
                {
                    int buf = b[ii];
                    b[ii] = b[jj];
                    b[jj] = buf;
                }
            }

        }
        int counter = 0;
        for (int ii = 0; ii < n; ii++)
        {
            if (b[ii] != c[ii])
            {
                counter++;
            }
            if (counter > 2)
            {
                Console.WriteLine("NO");
                return;
            }
        }




        if (counter == 2)
        {
            Console.WriteLine("YES");
        }
        else if (counter == 0)
        {
            Console.WriteLine("YES");
        }
        else
        {
            Console.WriteLine("NO");
        }

    }
}