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x,y=[int(i) for i in input().split()]
n=int(input())
t=[int(i) for i in input().split()]
def oblicz(x,y,k):
    if x>y:x,y=y,x
    while t[k]>x and k>0:k-=1
    xk=x//t[k]
    yk=y//t[k]
    if x==0 or y==0: return 0
    else:
        return (xk*yk)+min(oblicz(x-xk*t[k],y,k)+oblicz(xk*t[k],y-yk*t[k],k),oblicz(x,y-yk*t[k],k)+oblicz(x-xk*t[k],yk*t[k],k))
if x%t[0] or y%t[0]:
    print(-1)
else:
    print(oblicz(x,y,n-1))