#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	int n;
	cin >> n;

	deque<int> deq;
	rep(i, n){
		int a;
		cin >> a;
		deq.push_back(a);
	}

	int x = deq[0];
	while(!deq.empty() && deq.front() == x){
		deq.pop_front();
	}

	bool erased = false;
	while(!deq.empty() && deq.back() == x){
		erased = true;
		deq.pop_back();
	}

	if(deq.empty()){
		cout << 0 << endl;
		exit(0);
	}

	set<int> st;
	st.insert(x);
	x = deq.front();
	while(!deq.empty()){
		if(st.count(deq.front()) == 1){
			cout << -1 << endl;
			exit(0);
		}

		if(deq.front() != x){
			st.insert(x);
			x = deq.front();
		}
		
		deq.pop_front();
	}
	
	if(erased){
		cout << 1 << endl;
	}else{
		cout << 0 << endl;
	}
}