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 swap{
  public static void Main(){
    int n = int.Parse(Console.ReadLine());
    int[] num = new int[n];
    String[] str = Console.ReadLine().Split(' ');
    for(int i=0; i<n; i++){
      num[i] = int.Parse(str[i]);
    }
    count c = new count();
    c.asc(n,num);
    c.desc(n,num);
    if((c.getcntasc()<=2)|| (c.getcntdesc()<=2)){
      Console.WriteLine("YES");
    }
    else{
      Console.WriteLine("NO");
    }
  }
}

class count{
  int cntasc,cntdesc;
  public count(){
    cntasc = 0;
    cntdesc = 0;
  }
  public int getcntasc(){
    return cntasc;
  }
  public int getcntdesc(){
    return cntdesc;
  }
  public void asc(int n, int[] num){
    for(int i=n-1; i>=0; i--){
      if(num[i] != i+1){
        cntasc++;
      }
    }
  }
  public void desc(int n, int[] num){
    for(int i=0; i<n; i++){
      if(num[i] != i+1){
        cntdesc++;
      }
    }
  }
}