#include <bits/stdc++.h>
using namespace std;
int p[210];
int main() {
  int n, k;
  int t;
  int w;
  scanf("%d", &t);
  for (int m = 1; m <= t; m++) {
    scanf("%d%d", &n, &k);
    memset(p, 0, sizeof(p));
    int f = 0;
    int l = 0;
    for (int i = 1; i <= k; i++) {
      scanf("%d", &w);
      p[++l] = w - f - 1;
      f = w;
    }
    p[++l] = n - f;
    int maxx = 0;
    for (int i = 2; i < l; i++)
      if (p[i] > maxx) maxx = p[i];
    int maxxx = max(p[1], p[l]);
    maxx = ((maxx + 1) >> 1) + 1;
    maxxx += 1;
    printf("%d\n", max(maxx, maxxx));
  }
}
