#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
int s[202];
int main() {
  int t, n, k, x;
  scanf("%d", &t);
  while (t--) {
    int t = 0;
    scanf("%d%d", &n, &k);
    for (int i = 0; i < k; i++) scanf("%d", s + i);
    sort(s, s + k);
    int last = s[0];
    int maxl = last;
    for (int i = 1; i < k; i++) {
      maxl = max(maxl, (s[i] - last + 2) / 2);
      last = s[i];
    }
    maxl = max(maxl, n - last + 1);
    printf("%d\n", maxl);
  }
  return 0;
}
