#include <bits/stdc++.h>
const int M = 220;
int a[M];
int main() {
  int t;
  scanf("%d", &t);
  while (t--) {
    int n, m;
    scanf("%d%d", &n, &m);
    for (int i = 0; i < m; ++i) {
      scanf("%d", &a[i]);
    }
    int ans = a[0];
    for (int i = 1; i < m; ++i) {
      ans = std::max(ans, (a[i] - a[i - 1]) / 2 + 1);
    }
    printf("%d\n", std::max(ans, n - a[m - 1] + 1));
  }
}
