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def licz(size, pictures):
    if(size[0] == 0) | (size[1] == 0):
        return 0
    if len(pictures) == 0:
        print('error')
        return 0
    c = int(size[0] / pictures[0])
    r = int(size[1] / pictures[0])
    if(c == 0) | (r == 0):
        return licz(size, pictures[1:])
    return c * r + licz([size[0] - c * pictures[0], size[1]], pictures[1:]) + licz([c * pictures[0], size[1] - r * pictures[0]], pictures[1:])


w = list(map(int, input().split()))
n = int(input())
r = list(map(int, input().split()))

if (w[0] % r[0] != 0) | (w[1] % r[0] != 0):
    print(-1)
    exit()

r.reverse()
ile = licz(w, r)
print(ile)