#include <bits/stdc++.h>
using namespace std;
int a[205], max1, n, k, t;
int main() {
  cin >> t;
  while (t > 0) {
    t--;
    scanf("%d%d", &n, &k);
    for (int i = 0; i < k; i++) scanf("%d", &a[i]);
    max1 = 0;
    if (k == 1)
      cout << max(n - a[0] + 1, a[0]) << endl;
    else {
      for (int i = 1; i < k; i++) {
        if ((a[i] - a[i - 1]) / 2 + 1 > max1) max1 = (a[i] - a[i - 1]) / 2 + 1;
      }
      int d = max(n - a[k - 1] + 1, a[0]);
      if (d > max1)
        cout << d << endl;
      else {
        cout << max1 << endl;
      }
    }
  }
  return 0;
}
