You are given an array (a) of length (n). For an integer (k), we call it piggy if and only if it is possible to sort (a) in non-descending order by performing the following operation an arbitrary number of times (possibly zero): First, select two indices (i) and (j) ((1 \le i < j \le n)) such that (|a_i - a_j| \ge k); Then, swap (a_i) and (a_j). You need to determine the largest piggy integer (k). If such an integer does not exist, output (-1). Each test contains multiple test cases. The first line contains the number of test cases (t) ((1 \le t \le 10^4)). The description of the test cases follows. The first line of each test case contains a single integer (n) ((1 \le n \le 2 \cdot 10^5)) — the length of (a). The second line contains (n) integers (a_1,a_2,\ldots,a_n) ((1 \le a_i \le 10^9)) — the elements of (a). It is guaranteed that the sum of (n) over all test cases does not exceed (2\cdot10^5). For each test case, output a single integer — the largest piggy integer (k). If such an integer does not exist, output (-1) instead. In the first and the second test case, no matter how large (k) is, you can always sort (a) in non-descending order by not performing any operations. In the third test case, the largest piggy (k) is (2). We can select (i=2) and (j=3) in the first operation, since (|a_2-a_3|=|4-2|=2 \ge k), and (a) is sorted in non-descending order. It can be proven that no piggy integers larger than (2) exist.