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

#define int long long
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

signed main(){
	int n;
	cin >> n;
	vector<int> a(n);
	deque<int> b;
	b.push_front(0);
	for(int i = 0;i < n;i++){
		cin >> a[i];
		if(b.back() != a[i]){
			b.push_back(a[i]);
		}
	}
	b.pop_front();

	auto f = [](deque<int> &q){
		set<int> st;
		for(int i = 0;i < q.size();i++){
			if(st.find(q[i]) != st.end()){
				return false;
			}
			st.insert(q[i]);
		}
		return true;
	};

	if(f(b)){
		cout << 0 << endl;
	}else{
		int x = b.back();
		b.pop_back();
		if(b.front() != x){
			b.push_front(x);
		}
		if(f(b)){
			cout << 1 << endl;
		}else{
			cout << -1 << endl;
		}
	}

	return 0;
}