#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <fstream>
#include <iostream>
#include <map>
#include <queue>
#include <string>
#include <stack>
#include <vector>
#include <set>
#include <tuple>
#include <utility>
#include <functional>
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef tuple<int,int,int> T;
const int INF = 1000000000;
const int MOD = 1000000007;
int main(){
	int n;
	cin >> n;
	vector<int> a(n);
	for(int i = 0;i < n;i++)cin >> a[i];

	vector<int> b;
	for(int i = 0;i < n;i++){
		int j = i;
		while(j < n && a[j] == a[i]){
			j++;
		}
		b.push_back(a[i]);
		i = j-1;
	}

	int m = b.size();
	map<int,int> prev;
	for(int i = 0;i < m;i++){
		if(prev[b[i]] == 0)prev[b[i]] = i+1;
		else if(prev[b[i]] + 1 < i+1){
			if(!(prev[b[i]] == 1 && i+1 == m)){
				cout << -1 << endl;
				return 0;
			}
		}
	}

	if(m == 1)cout << 0 << endl;
	else{
		if(b[0] == b[m-1])cout << 1 << endl;
		else cout << 0 << endl;
	}

}