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 ProgramB
    {
        static void Main(string[] args)
        {

            string input = Console.ReadLine();
            int n = int.Parse(input);
            string[] input_p = Console.ReadLine().Split(' ');
            int[] p = new int[n];
            for(int i = 0;i < n;i++)
                p[i] = int.Parse(input_p[i]);

            int count = 0;
            
            for(int i = 0;i < n;i++)
            {
                if(p[i] != i + 1)
                    count++;
            }

            if( count == 2 || count == 0)
                Console.WriteLine("YES");
            else
                Console.WriteLine("NO");
        }
    }