#include <bits/stdc++.h>

using i64 = long long;

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

	int n;
	std::cin >> n;

	std::vector<int> a(n);
	for (int i = 0; i < n; i++) {
		std::cin >> a[i];
	}

	int d = (n + a[1] - a[0]) % n;
	for (int i = 1; i < n; i++) {
		if ((a[i - 1] + d) % n != a[i]) {
			std::cout << "-1\n";
			return 0;
		}
	}
	
	int Min = std::min_element(a.begin(), a.end()) - a.begin();
	if (d == 0) {
		std::cout << (a[Min] == 0 ? 0 : -1) << "\n";
		return 0;
	}

	std::cout << d + a[Min] + (Min == a[Min] ? 0 : 1) << "\n";

	return 0;
}