#include <bits/stdc++.h>
using namespace std;
const int mn = 205;
int rd() {
  int x = 0, f = 1;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-') f = -1, c = getchar();
  while (isdigit(c)) x = (x << 3) + (x << 1) + c - 48, c = getchar();
  return x * f;
}
inline void rt(int x) {
  x >= 10 ? rt(x / 10), putchar(x % 10 + 48) : putchar(x + 48);
}
int t = rd(), n, k, ans, a[mn];
int main() {
  while (t--) {
    n = rd(), k = rd();
    for (int i = (1); i <= (k); i++) a[i] = rd();
    sort(a + 1, a + k + 1);
    ans = a[1];
    for (int i = 1; i < k; i++) ans = max(ans, ((a[i + 1] - a[i]) >> 1) + 1);
    rt(max(ans, n - a[k] + 1)), putchar('\n');
  }
  return 0;
}
