#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t;
  cin >> t;
  while (t--) {
    int n;
    long long k;
    cin >> n >> k;
    long long arr[n], sum = 0;
    for (int i = 0; i < n; i++) cin >> arr[i];
    sort(arr, arr + n);
    if (arr[0] + k >= arr[n - 1] - k)
      cout << arr[0] + k << endl;
    else
      cout << -1 << endl;
  }
  return 0;
}
