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: Wrong Answer)

using System;
class Program
{
  public static void Main(){
        // Your code here!
        int n =int.Parse(System.Console.ReadLine());
        string[] s = System.Console.ReadLine().Split(' ');
        int[] p = new int[n];
        for(int i=0 ; i<n ; i++){
            p[i] = int.Parse(s[i]);
        }
        bool flug = false;
        for(int i=0 ; i<n-1 ; i++){
            if(p[i]>p[i+1]){
                if(flug){
                    System.Console.WriteLine("NO");
                    return;
                }
                int index = p[i]-1;
                int temp = p[index];
                p[index] = p[i];
                p[i] = temp;
                flug = true;
            }
        }
        System.Console.WriteLine("YES");
    }
}