def ile(h,w,d):
    i = h//d
    j = w//d
    return [i,j]

h, w = list(map(int,input().split()))
n = int(input())
a = list(map(int,input().split()))
if h%a[0] != 0 or w%a[0]!=0:
    print(-1)
else:
    a = a[::-1]
    dp = [[h,w]]
    ans = 0
    for i in range(n):
        d = a[i]
        ndp = []
        for p in dp:
            ch, cw = p
            k, l = ile(ch, cw, d)
            ans += k*l
            if (ch%d)*l!=0:
                ndp.append([ch%d, l*d])
            if k*(cw%d)!=0:    
                ndp.append([k*d, cw%d])
            if (ch%d)*(cw%d)!=0: 
                ndp.append([ch%d, cw%d])
            dp = ndp
    print(ans)