#include <bits/stdc++.h>
int main() {
  int t, n, k;
  int now, last = 0, max;
  std::cin >> t;
  while (t--) {
    std::cin >> n >> k;
    std::cin >> now;
    max = now - 0;
    last = now;
    while (--k) {
      std::cin >> now;
      last = (now - last) / 2 + 1;
      if (last > max) {
        max = last;
      }
      last = now;
    }
    if (n - now + 1 > max) max = n - now + 1;
    std::cout << max << "\n";
  }
  return 0;
}
