Mathematical challenge

Hi,
Today i have recd. mail send by our GM-Finance

GM-Finance say:-

What is your degree? Can you solve the attached file?
This is a mathematical challenge, and it's been said that:

If you're an engineer or mathematician (three seconds are enough), you should be able to solve it in (under) three minutes,
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.

What is the missing number in this logical series?
1, 2, 6, 42, 1806, ____???

The answer is the password to open the spreadsheet that is attached to
this email. If you figure it out, open the spreadsheet, type your name
in, save it and resend it to your friends.

(See attached file)

Note :- only two minute waste for solve this question---Rahul