IQ TEST♥

This is a mathematical challenge, and it's been said that:

這是項數學挑戰！有這麼一種說法：

If you're an engineer, you should be able to solve it in (under) three

minutes (more like 30 seconds), if you're an architect, in three hours; if

you're a doctor, in six hours; if you're an accountant, in three months and

if you're a lawyer, probably never.

你若是工程師，你應該可在三分鐘內（３０秒左右吧）解開這道題； 你若是建築

師，給你三小時；你若是醫生，給你六小時；你若是會計師，你可能永遠都答不出

What is the missing number in this logical series?

問題如下：空格中的數字為？

1, 2, 6, 42, 1806, ____???