solve this

This is a mathematical challenge, and it's been said that:
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.
if you're good at math or logical problems, solve this challenge and
the answer is the password to open the spreadsheet to which you can
add your name to the hall of fame.

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.
Good luck

the excel file, you can request to me..i'll send it..