River Crossing

Imagine a river. There are four boats on the right side of the river and you want to take all of them to the left side. Each boat takes a certain amount of time to cross the river. One boat takes 8 minutes, one takes 4 minutes, one takes 2 minutes and the last one takes 1 minute. You are allowed to take one boat of a smaller time and put it inside a boat of a larger time and take it across. So for example you can put the 4 minute boat inside the 8 minute boat and take it across for a time of 8 minutes. However, you are now stuck on the left side and must then take one of the boats back to the right side to pick up the remaining two boats. So if you took the 4 minute boat back, you would have a total of 12 minutes and only the 8 minute boat on the left side.  What is the shortest time it takes to get all four boats to the left side?