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;

namespace abc135_b
{
  class Program
  {
    static void Main(string[] args)
    {
      var n = CIn.ReadInt();
      var a = CIn.ReadIntArray(n);

      var diff = 0;

      for (var i = 0; i < n; i++)
      {
        if (a[i] != i + 1)
        {
          if (diff == 0)
          {
            diff = a[i];
          }
          else
          {
            if (diff == i + 1)
            {
              diff = -1;
            }
            else
            {
              Console.WriteLine("NO");
              return;
            }
          }
        }
      }
      
      if (diff == -1 || diff == 0)
      {
        Console.WriteLine("YES");
      }
      else
      {
        Console.WriteLine("NO");
      }
    }
  }

  class CIn
  {
    private static readonly Queue<string> fieldBuffer = new Queue<string>();

    public static string Read()
    {
      if (fieldBuffer.Count == 0)
      {
        foreach (var field in Console.ReadLine().Split())
        {
          fieldBuffer.Enqueue(field);
        }
      }

      return fieldBuffer.Dequeue();
    }

    public static int ReadInt() => int.Parse(Read());
    public static long ReadLong() => long.Parse(Read());
    public static double ReadDouble() => double.Parse(Read());

    public static string[] ReadStringArray(long n)
    {
      var array = new string[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = Read();
      }

      return array;
    }

    public static int[] ReadIntArray(long n)
    {
      var array = new int[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = ReadInt();
      }

      return array;
    }

    public static long[] ReadLongArray(long n)
    {
      var array = new long[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = ReadLong();
      }

      return array;
    }
    public static double[] ReadDoubleArray(long n)
    {
      var array = new double[n];

      for (var i = 0; i < n; i++)
      {
        array[i] = ReadDouble();
      }

      return array;
    }
  }
}