A 400-meter medley relay involves 4 different swimmers, who successively swim 100 meters of the backstroke, breaststroke, butterfly and freestyle. A coach has six very fast swimmers whose expected times in the individual events are given below:

How the coach assign swimmers to the competition so as to minimize the sum of their times?

Swimmer Backstroke Breaststroke Butterfly Freestyle 1 65 73 63 57 2 67 70 65 58 68 72 69 55 4 67 75 70 59 5 71 69 75 57 69 71 66 59