The Aether-Skiff of Corvanth Reach

Above Corvanth Reach, a chain of floating rocks called the Drift hangs in the sky, and the only way across is by aether-skiff. A skiff burns glowing aether-crystals to fly, and it can hold at most capacity crystals at once. At the start of every hop, the skiff is moored at a dock where it may take on crystals. The Skyfarers' Guild pays a bounty to whoever can tell a pilot, before launch, how many times the skiff will have to take on crystals along a given route.

The pilot follows one fixed rule:

• The skiff launches from the first dock carrying exactly capacity crystals. This initial load is not counted as a refill.
• Before each hop, if the crystals aboard are fewer than that hop costs, the skiff refills to exactly capacity crystals. This counts as one refill. If the crystals aboard are enough (including exactly enough), it does not refill.
• The hop's cost is then subtracted from the crystals aboard.

Write skiffRefills(capacity, hops).

• capacity is a positive integer.
• hops is a list of non-negative integers; hops[i] is the number of crystals burned on the i-th hop, in the order they are flown. It may be empty.

Return the number of refills the pilot makes over the whole route, as an integer. If any single hop costs more than capacity, the route cannot be flown at all; return -1 in that case.