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h, w = map(int, input().split())
n = int(input())
ds = reversed(list(map(int, input().split())))
result = 0
area = 0
for d in ds:
  h1 = h // d * d
  w1 = w // d * d
  area1 = h1 * w1
  result += (area1 - area) // d**2
  area = area1
if area == h * w:
  print(result)
else:
  print(-1)