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

int main(){
	int N;
	cin >> N;
	vector<int> s(N+1);
	for(int i=1;i<=N;i++){
		cin >> s[i];
	}
	s[0] = 0;
	int j;
	int sgn_count = 0;
	int w;
	for(int i=N;i>=0;i--){
		if(s[i] != i){
			for(j=0;j<=N;j++){
				if(s[j] == i){
					break;
				}
			}
			w = s[i];
			s[i] = s[j];
			s[j] = w;
			sgn_count++;
		}
	}
	if(sgn_count %2 == 0){
		cout << 1 << endl;
	} else {
		cout << -1 << endl;
	}
}