/*
*/

//#pragma comment(linker, "/STACK:16777216")
#define _CRT_SECURE_NO_WARNINGS

#include <fstream>
#include <iostream>
#include <string>
#include <complex>
#include <math.h>
#include <set>
#include <vector>
#include <map>
#include <queue>
#include <stdio.h>
#include <stack>
#include <algorithm>
#include <list>
#include <ctime>
#include <memory.h>
#include <assert.h>

#define y0 sdkfaslhagaklsldk
#define y1 aasdfasdfasdf
#define yn askfhwqriuperikldjk
#define j1 assdgsdgasghsf
#define tm sdfjahlfasfh
#define lr asgasgash

#define eps 1e-9
#define M_PI 3.141592653589793
#define bs 1000000007
#define bsize 512

const int N = 1000;

using namespace std;

long long l, r, ans;

long long gcd(long long a, long long b)
{
	while (a&&b)a > b ? a %= b : b %= a;
	return a + b;
}

int main(){
	//freopen("route.in","r",stdin);
	//freopen("route.out","w",stdout);
	//freopen("F:/in.txt", "r", stdin);
	//freopen("F:/output.txt", "w", stdout);
	ios_base::sync_with_stdio(0);
	//cin.tie(0);

	cin >> l >> r;

	ans = -1;

	for (long long i = l; i <= l + 1; i++)
	{
		for (long long j = r; j >= r - 1; j--)
		{
			if (i >= l&&i <= r&&j >= l&&j <= r&&gcd(i, j) == 1)
				ans = max(ans, j - i + 1);
		}
	}
	cout << ans << endl;

	//	cin.get(); cin.get();
	return 0;
}