I'm trying to make a temporary ban command, but I have another teeny tiny nearly insignificant problem that I can't find the solution to once again.

Temporary_Ban(mob/M in world)
set desc = "(person) Ban a player for a set amount of minutes."
set category = "Owner"
if(M == src)
usr << "You cannot ban yourself."
if(M.Host == 1)
usr << "You cannot ban the host."
var/tempban = input("How long to ban?(Integers only)","Temporary Ban")
banned += M.key
banned += M.client.address
world << "[src] has banned [M] ([M.key]) for [tempban] minutes."
var/banny = (tempban *= 600)

banned -= M.key
banned -= M.client.address

The line I'm having the problem with is once again bolded and italicised. It says " expression" but I have no idea how to fix it. Help?
tempban *= 600 should be tempban * 600
var/banny = (tempban *= 600)

Just change that to var/banny = (tempban * 600)

*= is for changing a variable to a multiple of itself like

//Double the gold of the player *= 2