Standard Deviation in Excel

[ChipsAhoya]

I know what STD is an all, but I'm not the best with Excel, and I was just wondering if anyone know how to input a standard deviation formula that would be weighted. In my sheet, I have one column with the win/loss for the session, and one column for the # of hours in the session.

I know I need to have it like (1/N)Exi form, but I have no idea how to program that... Do I need to have a separate column for (session i's hours) / (total hours)?

Thanks for any help...

have a nice day,
ChipsAhoya