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;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _135B
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int[] p = Console.ReadLine().Split(' ').Select(str => int.Parse(str)).ToArray();
            int[] sortedP = p.OrderBy(x => x).ToArray();
            int i = 0;
            int j = 0;
            int chigautokoro = 0;
            bool iNiDainyuuShitaka=false;
            bool ans = true ;

            for (int a = 0; a < n && chigautokoro <= 2; a++)
            {
                if (p[a] != sortedP[a])
                {
                    chigautokoro++;
                    if (!iNiDainyuuShitaka)
                    {
                        i = a;
                    }
                    else
                    {
                        j = a;
                    }
                }
            }

            if (chigautokoro == 0)
            {
                ans = true;
            }else if (chigautokoro == 2)
            {
                int a = p[i];
                p[i] = p[j];
                p[j] = a;

                for(int b = 0; b < n; b++)
                {
                    if (p[b] != sortedP[b])
                    {
                        ans = true;
                    }
                }
            }
            else
            {
                ans = false;
            }

            if (ans)
            {
                Console.WriteLine("YES");
            }
            else Console.WriteLine("NO");
        }
    }
}