#include <bits/stdc++.h>
using namespace std;
int main() {
  int t, n, k;
  cin >> t;
  int result[t];
  for (int i = 0; i < t; i++) {
    int maxSec = 0;
    cin >> n >> k;
    int tap[k];
    for (int j = 0; j <= k; j++) {
      if (j == k)
        tap[j] = n;
      else
        cin >> tap[j];
    }
    for (int j = 0; j < k; j++) {
      if (j == 0) maxSec = tap[j] - 1;
      if (j == (k - 1) && maxSec < tap[j + 1] - tap[j])
        maxSec = tap[j + 1] - tap[j];
      else if (maxSec < (tap[j + 1] - tap[j]) / 2)
        maxSec = (tap[j + 1] - tap[j]) / 2;
    }
    result[i] = maxSec + 1;
  }
  for (int i = 0; i < t; i++) cout << result[i] << endl;
  return 0;
}
