String Keygen() {
      Byte[] key = new Byte[16] { 4, 4, 0, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
      Byte[] primes = new Byte[4] { 2, 3, 5, 7 };
      while (true) {
      LOOP:
          for (Int32 i = 4; i < 16; i++)
              key[i] = (Byte)Random.Shared.Next(0, 10);
          if (key.Skip(4).Any(x => x == 4))
              continue;
          if (key.Distinct().Count() < 5)
              continue;
          if (key.Count(x => x == 8) < 3)
              continue;
          if (key.Count(x => primes.Contains(x)) < 2)
              continue;
          Int32 sum = key.Sum(x => x switch {
              0 => -14,
              1 => -12,
              2 => -8,
              3 => -7,
              4 => -3,
              5 => -1,
              6 => 2,
              7 => 4,
              8 => 8,
              9 => 9
          });
          if (55 <= sum && sum <= 110)
              continue;
          for (Int32 i = 0; i < 82; i++)
              sum = (sum + i) % 110;
          if (sum < 55)
              continue;
          for (Int32 i = 4; i <= 12; i += 4)
              for (Int32 j = 0; j < 3; j++)
                  if (key[i + j] == key[i + j + 1])
                      goto LOOP;
          break;
      }
      return key.Aggregate("", (a, b) => $"{a}{b}", s => s.Insert(12, "-").Insert(8, "-").Insert(4, "-"));
  }