#include<iostream>
#include<stdio.h>
#include<string>
#include<string.h>
#include<vector>
using namespace std;

#pragma warning(push)
#pragma warning(disable: 4996)

long long x, y;

long long x_initial, y_initial;

int gcd(int a, int b)
{
	if (!b) return a;
	return gcd(b, a % b);
}

int main()
{
	//freopen("fisier.in", "r", stdin);
	//freopen("fisier.out", "w", stdout);

	cin >> x_initial >> y_initial;

	x = x_initial;
	y = y_initial;

	if (gcd(x, y) == 1)
	{
		printf("%d", y - x + 1);
		return 0;
	}

	if (x % 2 == 0 && y % 2 == 0)
	{
		y--;
	}

	while (y > x)
	{
		y -= 2;

		if (gcd(x, y) == 1)
		{
			printf("%d\n", y - x + 1);
			return 0;
		}
	}

	x = x_initial;
	y = y_initial;

	if (x % 2 == 0 && y % 2 == 0)
	{
		x++;
	}

	while (x < y)
	{
		x += 2;

		if (gcd(x, y) == 1)
		{
			printf("%d\n", y - x + 1);
			return 0;
		}
	}

	printf("%d\n", -1);

	return 0;
}

#pragma warning(pop)