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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <vector> 
using namespace std;
#include <bits/stdc++.h>

int main(){
    // int N = 5;
    int N;
    cin >> N;

    // int pn[N] = {5,2,3,4,1};
    int pn[N+1];

	for(int i=1; i<N+1; i++) {
		int x;
		cin >> x;
        pn[i] = x;
	};
    
    // int max = 0;
    // int tmp_max = pn[0];

    // for(int i=1; i<N; i++) {   
    //     // cout << pn[i] << endl;
    //     // cout << pn[i-1] << endl;
    //     if(tmp_max > pn[i]){
    //         max = tmp_max;
    //         // cout << tmp_max << endl; 
    //     }
    //     else{
    //     }
    // }

    // cout << max <<endl;

    int ng_count=0;
    for(int i=1; i<N+1; i++){
        // cout << "i+1" << endl;
        // cout << i+1 << endl;
        
        // cout << "pn" << endl;
        // cout << pn[i] <<endl;
        if(i!=pn[i]){
            ng_count++;
        }
        else{

        }
    }

    // cout << ng_count << endl;

    if(ng_count<=2){
        cout<< "OK" <<endl;
    }
    else if(ng_count>2){
        cout << "NO" << endl;
    }
    else{}

    // for(int i; i<N; i++){}
}