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

if w % s[0] > 0 or h % s[0] > 0:
    print('-1')
    exit(0)

res = 0
while w > 0 and h > 0:
    w, h = min(w, h), max(w, h)
    res_c, r, f = 0, w, -1
    for v in s[::-1]:
        if v <= r:
            if f == -1: f = v
            res_c += (r // v) * (f // v)
            r %= v
            if r == 0: break
    
    res += res_c * (h // f)
    h %= f

print(res)