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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

import Data.List
import Data.Char
import qualified Data.ByteString.Char8 as BS8

main :: IO ()
main = do
    [n] <- unfoldr (BS8.readInt . BS8.dropWhile isSpace) <$> BS8.getLine
    pi <- unfoldr (BS8.readInt . BS8.dropWhile isSpace) <$> BS8.getLine
    let ideal = [(x, x) | x <- [1..n]]
        p = zip [1..n] pi
        result = ideal \\ p

    putStrLn $ if (length result) <= 2
        then "YES"
        else "NO"