#include <bits/stdc++.h>
using namespace std;
typedef struct water {
  int taps;
  int distance_left;
  int distance_right;
};
const long long array_maxn = 205;
water water_taps[array_maxn];
long long answer = -1;
int cmp(const water& a, const water& b) { return a.taps < b.taps; }
int main() {
  std::ios::sync_with_stdio(false);
  int T;
  cin >> T;
  while (T--) {
    int n, k;
    cin >> n >> k;
    answer = 0;
    for (register int i = 1; i <= k; i++) {
      cin >> water_taps[i].taps;
    }
    sort(water_taps + 1, water_taps + 1 + k, cmp);
    for (register int i = 1; i <= k; i++) {
      int x;
      if (i == 1) {
        x = abs(1 - water_taps[i].taps);
        if (x > answer) {
          answer = x;
        }
      }
      if (i > 1) {
        x = abs(water_taps[i].taps - water_taps[i - 1].taps) / 2;
        if (x > answer) {
          answer = x;
        }
      }
      if (i == k) {
        x = abs(n - water_taps[i].taps);
        if (x > answer) {
          answer = x;
        }
      }
    }
    cout << answer + 1 << endl;
  }
  return 0;
}
