Four guys are on the left side of a bridge and they need to go to the right side. It is night and the guys have just one electric torch light. Each guy has a different walking time. Namely, 10 second, 5 second, 2 second, and 1 second. At most 2 guys can be on the bridge at the same time, and when they walk in pair they have the speed of the slowest guy.
What it the minimum time to have the four guys on the right side of the bridge?