#include<iostream>
#include<vector>

using namespace std;
unsigned long long gcd(unsigned long long a,unsigned long long b)
{
	if (b == 0) return a;
	return gcd(b,a%b);
}
int main()
{
	unsigned long long a, b;
	unsigned long long max = 0;
	int ok = 0;
	cin >> a >> b;
	for (unsigned long long i = a;i < b - 1; ++i)
	{
		for (unsigned long long j = b; j>i; --j)
		{
			if (gcd(i, j) == 1 && j-i+1 > max)
			{
				max = j - i + 1;
			}
		}
	}
	if (max == 0) cout << "-1\n";
	else cout << max << "\n";
}