#include <bits/stdc++.h>
using namespace std;
const int maxn = 205;
vector<int> v;
bool a[maxn];
int main() {
  int t, n, k;
  cin >> t;
  while (t--) {
    cin >> n >> k;
    memset(a, false, sizeof(a));
    v.clear();
    for (int i = 0; i < k; i++) {
      int u;
      cin >> u;
      a[u] = true;
      v.push_back(u);
    }
    int ans = 0;
    for (int i = 1; i <= n; i++) {
      if (!a[i]) {
        int temp = maxn;
        for (int j = 0; j < v.size(); j++) {
          int u = v[j];
          temp = min(temp, abs(u - i));
        }
        if (temp != maxn) ans = max(ans, temp);
      }
    }
    cout << ans + 1 << endl;
  }
  return 0;
}
