import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int t = sc.nextInt();
        String result = IntStream.range(0, t).mapToObj(a -> {
            int n = sc.nextInt();
            int[] values = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                values[i] = sc.nextInt();
            }
            boolean first = true;
            for (int i = 1; i < n && first; i++) {
                first = values[i] < values[i + 1];
            }
            if (first) {
                return "0";
            }
            long left = 0;
            long right = Integer.MAX_VALUE;
            while (right - left > 1) {
                long m = (left + right) / 2;
                boolean disable = false;
                for (int i = 1; i < n && !disable; i++) {
                    disable = values[i] + i * m >= values[i + 1] + (i + 1) * m;
                }
                if (disable) {
                    left = m;
                } else {
                    right = m;
                }
            }
            boolean enable = true;
            for (int i = 1; i < n && enable; i++) {
                enable = values[i] + i * right < values[i + 1] + (i + 1) * right;
            }
            return String.valueOf(enable ? right : -1);
        }).collect(Collectors.joining("\n"));
        System.out.println(result);
    }
}
class Scanner {
    BufferedReader br;
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() {
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            
        }
    }
    
    public int nextInt() {
        return Integer.parseInt(next());
    }
    
    public long nextLong() {
        return Long.parseLong(next());
    }
    
    public double nextDouble() {
        return Double.parseDouble(next());
    }
    
    public String next() {
        try {
            while (!st.hasMoreTokens()) {
                st = new StringTokenizer(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return st.nextToken();
        }
    }
    
}