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;

namespace B___0_or_1_Swap
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());

            string[] w = Console.ReadLine().Split();

            int[] p = new int[n + 1];
            p[0] = 0;

            int swap1 =0,swap2=0;
            for(int i=1;i<n+1;i++)
            {
                int nw = int.Parse(w[i-1]);
                if(i == nw)
                {
                    p[i] = nw;
                }
                else
                {
                    if (swap1 == 0)
                    {
                        swap1 = nw;
                    }
                    else
                    {
                        swap2 = nw;
                    }
                }
            }

            if (swap1 > 0)
            {
                p[swap1] = swap1;
            }
            if (swap2 > 0)
            {
                p[swap2] = swap2;
            }
           
            for(int i =1;i<n+1;i++)
            {
                if(p[i] != i)
                {
                    Console.WriteLine("NO");
                    Environment.Exit(0);
                }
            }
            Console.WriteLine("YES");
        }
    }
}