#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  bool arrGarden[201] = {0};
  int nTest, i, j, k, nTaps, iLength, iTemp, iSecond;
  vector<int> arrTaps;
  cin >> nTest;
  for (i = 1; i <= nTest; ++i) {
    cin >> iLength >> nTaps;
    for (j = 1; j <= nTaps; ++j) {
      cin >> iTemp;
      arrTaps.push_back(iTemp - 1);
    }
    for (iSecond = 1;; ++iSecond) {
      for (j = 0; j <= nTaps - 1; ++j) {
        for (k = max(arrTaps[j] - iSecond + 1, 0);
             k <= min(200, arrTaps[j] + iSecond - 1); ++k) {
          arrGarden[k] = true;
        }
      }
      if (count(arrGarden, arrGarden + iLength, false) == 0) {
        cout << iSecond << endl;
        goto reset;
      }
    }
  reset:
    for (j = 0; j <= 200; ++j) {
      arrGarden[j] = false;
    }
    arrTaps.clear();
  }
endapp:
  return 0;
}
