#include <bits/stdc++.h>
#ifdef LOCAL
#include "./debug.cpp"
#else
#define debug(...)
#define print_line
#endif
using namespace std;
using ll = long long;

bool check(const vector<ll> &a) {
    bool ok = true;
    for (int i = 0; i < a.size() - 1; i++) ok &= a[i] < a[i + 1];
    return ok;
}

int main() {
    int T;
    cin >> T;

    while (T--) {
        int N;
        cin >> N;
        vector<ll> A(N);
        for (int i = 0; i < N; i++) cin >> A[i];

        if (check(A)) {
            cout << 0 << '\n';
            continue;
        }

        ll lo = -1, hi = 2e9;
        while (hi - lo > 1) {
            ll mid = (lo + hi) / 2;

            vector<ll> B(N);
            for (int i = 0; i < N; i++) B[i] = A[i] + mid * i;
            if (check(B)) {
                hi = mid;
            } else {
                lo = mid;
            }
        }

        if (lo == -1) {
            cout << -1 << '\n';
        } else {
            cout << hi << '\n';
        }
    }
}