#include <bits/stdc++.h>
using namespace std;
int main() {
  long long t;
  cin >> t;
  while (t--) {
    long long n, k, i;
    cin >> n >> k;
    long long a[n], s = INT_MAX, l = INT_MIN;
    for (i = 0; i < n; i++) {
      cin >> a[i];
      s = min(s, a[i]);
      l = max(l, a[i]);
    }
    if (s + k >= l - k)
      cout << s + k << endl;
    else
      cout << "-1" << endl;
  }
  return 0;
}
