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 ClassLibrary1
{
    public class ListNode
    {
        public int val;
        public ListNode next;
        public ListNode(int x) { val = x; }
    }


    public class Class1
    {
        static List<string> cons = new List<string>();
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());

            string[] str = Console.ReadLine().Split(' ');
            int[] arr = new int[n];

            int[] arr2 = new int[n];

            for (int i = 0; i < n; i++)
            {
                arr[i] = int.Parse(str[i]);
            }

            arr2[0] = arr[0];

            int reversed1 = -1;
            int reversed2 = -1;
            for (int i = 1; i < n; i++)
            {
                arr2[i] = arr[i];
                if (arr[i - 1] > arr[i])
                {
                    if (reversed1 == -1)
                    {
                        reversed1 = i - 1;
                    }
                    else if (reversed2 == -1)
                    {
                        reversed2 = i;
                    }
                    else
                    {
                        Console.WriteLine("NO");
                        return;
                    }
                }
            }

            if (reversed1 == -1)
            {
                Console.WriteLine("YES");
                return;
            }
            else
            {
                if (reversed2 == -1)
                {
                    reversed2 = reversed1+1;
                }

                arr2[reversed1] = arr[reversed2];
                arr2[reversed2] = arr[reversed1];
            }

            for (int i = 1; i < n; i++)
            {
                if (arr2[i - 1] > arr2[i])
                {
                    Console.WriteLine("NO");
                    return;
                }

            }
            Console.WriteLine("YES");

        }

    }
}