#include <bits/stdc++.h>
using namespace std;
int arr[205];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    int n, k;
    cin >> n >> k;
    for (int i = 1; i <= k; i++) {
      cin >> arr[i];
    }
    int ma = 0;
    for (int i = 2; i <= k; i++) {
      ma = max(ma, (arr[i] - arr[i - 1] + 2) / 2);
    }
    ma = max(ma, arr[1]);
    ma = max(ma, n + 1 - arr[k]);
    cout << ma << '\n';
  }
}
