3 1 1 5//三头牛开车过一个通道.当一个牛进入通道时，它的速度V会变成V-D*X(X代表在它前面有多少牛),它减速后，速度不能小于L
There are three cows with one lane to drive on, a speed decrease
of 1, and a minimum speed limit of 5.
Two cows are possible, by putting either cow with speed 5 first and the cow
with speed 7 second.