#include <bits/stdc++.h>
using namespace std;
int a[110], b[110];
int main() {
  int t, n, k, i;
  scanf("%d", &t);
  while (t--) {
    int time = 0;
    memset(b, 0, sizeof(b));
    scanf("%d%d", &n, &k);
    for (i = 1; i <= k; i++) {
      scanf("%d", &a[i]);
      time = max(time, (a[i] - a[i - 1]) / 2 + 1);
    }
    time = max(time, n - a[k] + 1);
    time = max(time, a[1]);
    printf("%d\n", time);
  }
  return 0;
}
