#include <bits/stdc++.h>
using namespace std;
#define modulo 1000000007
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 10000000000000

bool is_ok(deque<int> A){
	map<int,int> mp;
	int now = 0;
	for(int i=0;i<A.size();i++){
		if(mp.count(A[i]))continue;
		mp[A[i]]=now;
		now++;
	}
	
	for(int i=0;i<A.size();i++)A[i] = mp[A[i]];
	auto B = A;
	sort(B.begin(),B.end());
	return A==B;
}

int main(){
	
	int N;
	cin>>N;
	
	deque<int> A(N);
	for(int i=0;i<N;i++)cin>>A[i];
	
	if(is_ok(A)){
		cout<<0<<endl;
		return 0;
	}
	
	for(int i=0;i<N;i++){
		if(A.back()!=A.front())break;
		int t = A.back();
		A.pop_back();
		A.push_front(t);
	}
	
	if(is_ok(A))cout<<1<<endl;
	else cout<<-1<<endl;
	
	return 0;
}