The Tollkeeper of Thistle Bridge

The tollkeeper of Thistle Bridge keeps a small ledger nailed beside her stool. It lists what each kind of cart owes to cross: an ox-cart three coins, a hand-barrow one, a pilgrim's pack nothing at all. Anything not written in the ledger is charged the stranger's rate of 5 coins.

This morning a line of carts is waiting, and she wants the day's total before she lifts the bar.

You are given two arguments:

• tolls — a dictionary (object) mapping a cart type (string) to its toll (a non-negative integer).
• carts — a list (array) of cart-type strings, in the order they wait at the bridge.

Write bridgeToll(tolls, carts), which returns an integer: the sum of the tolls for every cart in carts. A cart whose type appears in tolls pays exactly the amount listed there. A cart whose type does not appear in tolls pays 5. Every cart in the list pays, even if the same type appears many times. An empty carts list totals 0.