import java.util.*;
import java.math.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BigInteger n = new BigInteger(sc.next());
        BigInteger m = new BigInteger(sc.next());
        HashSet<BigInteger> set = new HashSet<>();
        while (true) {
            BigInteger mod = n.mod(m);
            if (mod.equals(BigInteger.ZERO)) {
                System.out.println(getLastNum(n.divide(m)));
                return;
            }
            n = mod;
            if (set.contains(n)) {
                System.out.println(-1);
                return;
            }
            set.add(n);
            n = n.multiply(BigInteger.TEN);
        }
    }
    
    static long getLastNum(BigInteger b) {
        long x = b.longValue();
        while (x > 0) {
            if (x % 10 != 0) {
                return x % 10;
            }
            x /= 10;
        }
        return 0;
    }
}