#include <bits/stdc++.h>
using namespace std;
int t;
int n, k;
int x[200 + 5];
int main() {
  scanf("%d", &t);
  while (t--) {
    scanf("%d%d", &n, &k);
    for (int i = 1; i <= k; i++) {
      scanf("%d", &x[i]);
    }
    int mx = x[1];
    for (int i = 2; i <= k; i++) {
      mx = max(mx, (x[i] - x[i - 1]) / 2 + 1);
    }
    mx = max(mx, n - x[k] + 1);
    cout << mx << endl;
  }
  return 0;
}
