#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);
    bool flag = 0;
    for (int i = 0; i < n; ++i) {
        std::cin >> a[i];
    }
    for (int i = 1; i < n; ++i) {
        if (a[i] != a[i - 1]) {
            flag = 1;
        }
    }
    if (!flag) {
        if (a[0] == 0) {
            std::cout << "0\n";
        } else {
            std::cout << n + 1 << "\n";
        }
        return 0;
    }
    bool ok = 0;
    int d = (a[1] - a[0] + n) % n;
    for (int i = 2; i < n; ++i) {
        if ((a[i] - a[i - 1] + n) % n != d) {
            ok = 1;
            break;
        }
    }
    if ((a[0] - a[n - 1] + n) % n != d) {
        ok = 1;
    }
    if (ok) {
        std::cout << "-1\n";
    } else {
        if (a[0] != 0) {
            std::cout << 1 + d << "\n";
        } else {
            std::cout << d << "\n";
        }
    }
    return 0;
}
// 1 3 2 4 5 0
// 0 1 2 3 4 5