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 Ruby (2.3.3) (judge verdict: Accepted)

def main(argv)
  n = gets.chomp.to_i
  p = gets.chomp.split(' ').map(&:to_i)
  
  swapped = false
  flag = true
  index_pair = [-1, -1]
  n.times do |i|
    if p[i] != (i + 1) then
      if swapped then
        if i != index_pair[1] then
          flag = false
          break
        end
      else
        j = p[i] - 1
        index_pair = [i, j]
        if p[j] == (i + 1) then
          swapped = true
        else
          flag = false
          break
        end
      end
    end
  end
  
  puts (flag ? 'YES' : 'NO')
end

if self.to_s == 'main' then
  main(ARGV)
end