#include <bits/stdc++.h>
using namespace std;
bool TAB[207];
queue<int> Q;
int main() {
  int t;
  scanf("%d", &t);
  while (t--) {
    int n, k;
    scanf("%d%d", &n, &k);
    int best = 0;
    int last = 0;
    for (int i = 0; i < k; i++) {
      int a;
      scanf("%d", &a);
      if (last == 0) {
        best = max(best, a);
      } else {
        best = max(best, (a - last) / 2 + 1);
      }
      last = a;
    }
    best = max(n - last + 1, best);
    cout << best << "\n";
  }
}
