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import sys
h, w = [int(x) for x in input().split()]
n = int(input())
d = [int(x) for x in input().split()]
if (h % d[0]) or (w % d[0]):
    print("-1")
    sys.exit(0)

current_cover = [0, 0]
result = 0
for a in d[::-1]:
    new_to_add_x = (h - current_cover[0]) // a
    new_to_add_y = (w - current_cover[1]) // a
    result += new_to_add_x * new_to_add_y
    result += (current_cover[0] // a) * new_to_add_y
    result += (current_cover[1] // a) * new_to_add_x
    current_cover[0] += new_to_add_x * a
    current_cover[1] += new_to_add_y * a

print(result)