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 static System.Console;
using static System.Math;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TemplateAlgorithm
{
    class Program
    {
        public const long Inf = (long)1e18;
        public const double eps = 1e-6;
        public const string alfa = "abcdefghijklmnopqrstuvwxyz";
        public const string ALFA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        public const int MOD = 1000000007;


        static void Main(string[] args)
        {
            int n = IO.ReadInt();
            var a  = IO.ReadIntArr();
            
            var d = a.Where((pi, i) => pi != i + 1).Count();
            Console.WriteLine(d == 0 || d == 2 ? "YES" : "NO");
        }

        static int ToInt(char c) => c - '0';

        #region MIN,MAX

        /// <summary>
        /// 最大値を返す
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="vals"></param>
        /// <returns></returns>
        public static T MaxValue<T>(params T[] vals) => vals.Max();

        /// <summary>
        /// 最小値を返す
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="vals"></param>
        /// <returns></returns>
        public static T MinValue<T>(params T[] vals) => vals.Min();
        #endregion
    }

    #region IO

    /// <summary>
    /// IO
    /// </summary>
    public class IO
    {
        public static string ReadStr(bool isTrim = false) => (isTrim) ? ReadLine().Trim() : ReadLine();
        public static int ReadInt() => int.Parse(ReadStr());

        public static long ReadLong() => long.Parse(ReadStr());

        public static decimal ReadDecimal() => decimal.Parse(ReadStr());
        public static double ReadDouble() => double.Parse(ReadStr());
        public static string[] ReadStrArr(char separator = ' ') => ReadLine().Split(separator);
        public static int[] ReadIntArr() => Array.ConvertAll(ReadStrArr(), int.Parse);
        public static long[] ReadLongArr() => Array.ConvertAll(ReadStrArr(), long.Parse);

        public static decimal[] ReadDecimalArr() => Array.ConvertAll(ReadStrArr(), decimal.Parse);
        public static double[] ReadDoubleArr() => Array.ConvertAll(ReadStrArr(), double.Parse);

        public static void Cout<T>(T value) => WriteLine(value);
        public static void CoutJoin<T>(T[] values, string separator = " ") => Cout(string.Join(separator, values));
    }

    #endregion    
}