package no025;

import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigDecimal n = new BigDecimal(sc.next());
		BigDecimal m = new BigDecimal(sc.next());
		try {
			BigDecimal ans = n.divide(m);
			ans = ans.stripTrailingZeros();
			String s = ans.unscaledValue().toString();
			System.out.println(s.charAt(s.length()-1));
		} catch (ArithmeticException e) {
			System.out.println(-1);
		}
	}
}