import java.math.BigInteger;
import java.util.Scanner;

/**
 * Created by alex on 12.02.2016.
 */
public class prog {

    public static void main(String args[]) {

        Scanner in = new Scanner(System.in);

        BigInteger A, B;
        A = in.nextBigInteger();
        B = in.nextBigInteger();

        BigInteger sol = BigInteger.valueOf(-1);

        for (int i = 0; i < 120; ++i)
            for (int j = 0; j < 120; ++j)
            {
                BigInteger C = A;
                BigInteger D = B;

                C = C.subtract(BigInteger.valueOf(i));
                D = D.subtract(BigInteger.valueOf(j));

                if (C.compareTo(BigInteger.ZERO) > 0 && D.compareTo(BigInteger.ZERO) > 0)
                {
                    BigInteger E = C.subtract(D);
                    E = E.abs();

                    if (C.gcd(D).compareTo(BigInteger.valueOf(1)) == 0)
                        sol = sol.max(E.add(BigInteger.ONE));

                }
            }

        System.out.print(sol);

    }
}