#include "bits/stdc++.h"
using namespace std;
//#include "atcoder/all"
//using namespace atcoder;
#define int long long
#define REP(i, n) for (int i = 0; i < (int)n; ++i)
#define RREP(i, n) for (int i = (int)n - 1; i >= 0; --i)
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define RFOR(i, s, n) for (int i = (int)n - 1; i >= s; --i)
#define ALL(a) a.begin(), a.end()
#define IN(a, x, b) (a <= x && x < b)
template<class T>istream&operator >>(istream&is,vector<T>&vec){for(T&x:vec)is>>x;return is;}
template<class T>inline void out(T t){cout << t << "\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout << t << " ";out(ts...);}
template<class T>inline bool CHMIN(T&a,T b){if(a > b){a = b;return true;}return false;}
template<class T>inline bool CHMAX(T&a,T b){if(a < b){a = b;return true;}return false;}
constexpr int INF = 1e18;

signed main(){
	int N;
	cin >> N;
	vector<int>a, b(N);
	cin >> b;
	REP(i, N) {
		if(a.empty() || a.back() != b[i]) a.emplace_back(b[i]);
	}
	map<int, int> mp;
	for(auto e: a) mp[e]++;
	for(auto[val, count]: mp) {
		if(count >= 3) {
			out(-1);
			return 0;
		}
	}
	for(auto[val, count]: mp) {
		if(count == 2) {
			if(val == a[0] && val == a.back()) continue;
			out(-1);
			return 0;
		}
	}
	if(a.size() >= 2 && a[0] == a.back()) out(1);
	else out(0);
}