#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>
#include <numeric>
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
constexpr long long MAX = 5100000;
constexpr long long INF = 1LL << 60;
constexpr int inf = 1000000007;
constexpr long long mod = 1000000007LL;
//constexpr long long mod = 998244353LL;
const long double PI = acos((long double)(-1));

using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;

bool check(vector<int> a) {
	auto b = a;
	auto c = a;
	b.erase(unique(b.begin(), b.end()), b.end());
	sort(c.begin(), c.end());
	c.erase(unique(c.begin(), c.end()), c.end());
	return b.size() == c.size();
}

int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/
	int n; scanf("%d", &n);
	vector<int> a(n); for (int i = 0; i < n; i++) scanf("%d", &a[i]);
	if (check(a)) { puts("0"); return 0; }
	vector<int> t;
	while (!a.empty() and a[0] == a.back()) {
		t.push_back(a.back());
		a.pop_back();
	}
	reverse(a.begin(), a.end());
	for (int i = 0; i < t.size(); i++) a.push_back(t[i]);
	reverse(a.begin(), a.end());
	if (check(a)) puts("1");
	else puts("-1");
	return 0;
}