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

def solve(w, h):
    if w == 0 or h == 0:
        return 0
    for i in reversed(obrazy):
        if i <= w and i <= h:
            a, b = divmod(w, i)
            c, d = divmod(h, i)
            return solve(b, h) + solve(i, d) * a + a * c
    else:
        raise "NUH UH"

try:
    print(solve(w, h))
except:
    print(-1)