Maximum Units on a Truck

You are given boxTypes where boxTypes[i] = [numberOfBoxes_i, numberOfUnitsPerBox_i] and truckSize (max number of boxes). Choose boxes to maximize total units on the truck.

maxUnits([[1,3],[2,2],[3,1]], 4) => 8

maxUnits([[5,10],[2,5],[4,7],[3,9]], 10) => 91