#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
const unsigned long long MAX = LONG_MAX;
void fasterIO() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
int sumDig(int a) {
  int sum = 0;
  while (a != 0) {
    sum += a % 10;
    a /= 10;
  }
  return sum;
}
int main() {
  int test;
  cin >> test;
  while (test--) {
    long long p, k;
    long long arr[150];
    cin >> p >> k;
    for (unsigned long long i = 0; i < (p); i++) cin >> arr[i];
    sort(arr, arr + p);
    if (abs((arr[0] + k) - arr[p - 1]) > k)
      cout << "-1" << endl;
    else
      cout << arr[0] + k << endl;
  }
  return 0;
}
