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 (GCC 5.4.1) (judge verdict: Accepted)

#include<stdio.h>
int main(void)
{
  int n;
  int p[50];
  int i,j;
  int a,b;
  int x,y;
  int c;
  int sum=0;
  
  scanf("%d",&n);
  for(i=0;i<n;i++)
    scanf("%d",&p[i]);
  
  a=-1;
  b=-1;
  
  for(i=0;i<n;i++)
  {
    if(p[i]!=i+1)
    {
      a=i;
      x=p[i];
      
      break;
    }
  }
  for(i=a+1;i<n;i++)
  {
    if(p[i]!=i+1)
    {
      b=i;
      y=p[i];
      
      break;
    }
  }
  if(a==-1&&b==-1)
    printf("YES");
  else{
  if(b==-1)
    printf("NO");
  else
  {
    c=p[a];
    p[a]=p[b];
    p[b]=c;
    for(i=0;i<n;i++)
    {
      if(p[i]!=i+1)
        break;
      sum++;
    }
    if(sum==n)
      printf("YES");
    else
      printf("NO");
  }
  }
  return 0;
}