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 up(int *a, int n){
  for(int i = 0; i < n - 1; i++){
    if(a[i + 1] - a[i] < 0){
      return 0;
    }
  }
  return 1;
}

int main(){
  int n;
  scanf("%d", &n);
  int p[n];
  for(int i = 0; i < n; i++){
    scanf("%d", &p[i]);
  }
  int flag = 0;
  if(up(p, n)){
    flag = 1;
  }
  int q[n];
  for(int i = 0; i < n; i++){
    for(int j = 0; j < n; j++){
      if(i != j){
        for(int k = 0; k < n; k++){
          q[k] = p[k];
        }
        int sw = q[i];
        q[i] = q[j];
        q[j] = sw;
        if(up(q, n)){
          flag = 1;
        }
      }
    }
  }
  if(flag){
    printf("YES\n");
  }else{
    printf("NO\n");
  }
  return 0;
}