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 Program
{
  static void Main()
  {
    int N = int.Parse(Console.ReadLine());
    string[] p = Console.ReadLine().Split(' ');
    string ans = "YES";
    int htr = -1;
    bool switched = false;
    for (int i = 0; i < N; i++)
    {
      int pi = int.Parse(p[i]);
      if (pi != i + 1)
      {
        if (!switched && int.Parse(p[pi - 1]) == i + 1)
        {
          switched = true;
          htr = pi - 1;
        }
        else if (i != htr)
        {
          ans = "NO";
          break;
        }
      }
    }
    
    Console.WriteLine(ans);
  }
}