#include <bits/stdc++.h>
const int MAXN = 200 + 5;
int t[MAXN], n, k;
void solve() { ; }
void init() {
  scanf("%d%d", &n, &k);
  int m = 0;
  t[0] = 1;
  t[k + 1] = n;
  for (int i = 1; i <= k + 1; i++) {
    if (i <= k) scanf("%d", &t[i]);
    int temp = t[i] - t[i - 1] + 1;
    if (!(i == 1 || i == k + 1)) temp = (temp + 1) / 2;
    m = (m >= temp ? m : temp);
  }
  int ans = m;
  printf("%d\n", ans);
}
int main(void) {
  int times;
  scanf("%d", &times);
  while (times--) init();
  return 0;
}
