#include <bits/stdc++.h>
using namespace std;
int main() {
  int t, n, k, i, j, res[201], max = 0, diff;
  cin >> t;
  for (i = 0; i < t; i++) {
    cin >> n >> k;
    int x[k];
    for (j = 0; j < k; j++) {
      cin >> x[j];
      if (j == 0) {
        max = x[j];
      }
      if ((j > 0) && (j < k)) {
        diff = (x[j] - x[j - 1]) / 2 + 1;
      }
      if (diff > max) {
        max = diff;
      }
      if (j == (k - 1)) {
        if ((n - x[j] + 1) > max) {
          max = n - x[j] + 1;
        }
      }
    }
    res[i] = max;
    diff = 0;
  }
  for (i = 0; i < t; i++) {
    cout << res[i] << "\n";
  }
  return 0;
}
