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;
using System.Linq;
using System.Threading.Tasks;

class Program {
    static void Main (string[] args) {
        int n  = int.Parse(Console.ReadLine ());
        string[] p = Console.ReadLine ().Split(' ');
        int[] pn = new int[n];
        int[] pn_sort = new int[n];
        int cnt = 0;

        for(int i=0;i<n;i++){
            pn[i] = int.Parse(p[i]);
            pn_sort[i] = int.Parse(p[i]);
        }

        Array.Sort(pn_sort);

        for(int i=0;i<n;i++){
            if(pn[i] != pn_sort[i]) cnt++;
        }

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

    }
}