import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

from math import gcd

a,b = map(int,read().split())

g = gcd(a,b); a //= g; b //= g

d = 10 ** 100
if d % b != 0:
    print(-1)
    exit()

x = a * d // b

while x % 10 == 0:
    x //= 10

answer = x % 10
print(answer)