Calculating the requirement of cartons based on the sum of two different size boxes

Column A contains number of Box Type X used
Column B contains number of Box Type Y used (1/2 size of box Type B)

The carton can take a combination of:
2 x Box Type X and 1 x Box Type Y
1 x Box Type X and 3 x Box Type Y
5 x Box Type Y on their own.

How do I calculate the number of cartons required in Column C based on the totals in Columns A and B.