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>
#include <math.h>
#include <stdlib.h>
//40min
int main(void){
  int n;
  int p[51];
  int t[51];
  int tmp;
  int i = 0, j = 0;

  scanf("%d", &n);

  for(int cnt = 1; cnt <= n; cnt++){
    scanf("%d", &p[cnt]);  
  }

  for(int cnt = 1; cnt <= n; cnt++){
    t[cnt] = p[cnt];
  }
  /*
  //昇順作る
  for(int a = 1; a <= n; a++){
    for(int b = a; b <= n; b++){
      if(t[a] > t[b]){
	tmp = t[a];
	t[a] = t[b];
	t[b] = tmp;
      }
    }
  }
  */
  for(int cnt = 1; cnt <= n; cnt++){
    //    printf("cnt = %d  p[cnt]=%d\n", cnt, p[cnt]);
    if(cnt != p[cnt]){
      if(i == 0){
	i = cnt;
      }else if(j == 0){
	j = cnt;
      }else{
       break;
      }
    }
  }

  if(i != 0 && j != 0){
    tmp = p[i];
    p[i] = p[j];
    p[j] = tmp;
  }
  
  // 昇順と同じこと確認する
  for(int cnt = 1; cnt <= n; cnt++){
    //    printf("cnt = %d  p[cnt]=%d\n", cnt, p[cnt]);
    if(cnt != p[cnt]){
      printf("NO\n");
      return 0;
    }
  }

  printf("YES\n");
  
  return 0;
}