#include <iostream>
#include <algorithm>
#include <vector>
#include <set>

void solve() {
    int n;
    std::cin >> n;

    std::vector<int> xs(n);
    for (auto& x : xs) std::cin >> x;

    int ans = 0;

    xs.erase(std::unique(xs.begin(), xs.end()), xs.end());
    if ((int)xs.size() > 1 && xs.front() == xs.back()) {
        ans = 1;
        xs.pop_back();
    }

    std::set<int> s;
    for (auto x : xs) {
        if (s.count(x)) ans = -1;
        s.insert(x);
    }

    std::cout << ans << "\n";
}

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

    solve();

    return 0;
}