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.Text;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static void Main(string[] args) {
            //string[] str = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得
            //int a = int.Parse(str[0]);  //数値で受け取りたい場合は変換する
            //int b = int.Parse(str[1]);
            //if ((a + b) % 2 == 0) {
            //    Console.WriteLine((a + b) / 2);
            //} else {
            //    Console.WriteLine("IMPOSSIBLE");
            //}


            //数列pのNi+1 - Niの差がすべて１ならOK、つまりN - 1ならOK
            //

            string str = Console.ReadLine();
            int n = int.Parse(str);
            string[] strData = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得

            var num = new List<int>();
            var num1 = new List<int>();

            var countDiff = 0;
            bool isPossible;

            foreach(var obj in strData) {
                num.Add(int.Parse(obj));
                num1.Add(int.Parse(obj));
            }

            num.Sort();

            //Console.WriteLine("[{0}]", string.Join(", ", num));
            //Console.WriteLine("[{0}]", string.Join(", ", num1));

            for(var i = 0; i < n; i++) {
                if(num[i] != num1[i]) {
                    countDiff++;
                }
            }

            if(countDiff <= 2) {
                isPossible = true;
            } else {
                isPossible = false;
            }


            if (isPossible) {
                Console.WriteLine("YES");
            } else {
                Console.WriteLine("NO");
            }

            Console.ReadKey();

        }
    }
   
}