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from typing import List


def tile(h: int, w: int, seq: List[int]) -> int:
    if h == 0 or w == 0:
        return 0

    if h % seq[0] != 0 or w % seq[0] != 0:
        return -1

    if len(seq) == 1:
        return (h // seq[0]) * (w // seq[0])

    fit_h = h // seq[1]
    fit_w = w // seq[1]

    new_h = fit_h * seq[1]
    new_w = fit_w * seq[1]

    tile_smaller = tile(new_h, new_w, seq[1:])

    return tile_smaller + (h * w - new_h * new_w) // (seq[0] * seq[0])
    

h, w = [int(x) for x in input().split(' ')]
n = int(input())

sizes = [int(x) for x in input().split(' ')]

print(tile(h, w, sizes))