#include <iostream>
#include <cstdlib>
using namespace std;


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

int main()
{
    long long a, b, i, j, lg, lung;
    cin >> a >> b;

    for(lung = b - a + 1 ; lung  ; lung--)
    {
        for(i = a ; i + lung - 1 <= b ; i++)
        {
            j = i + lung - 1;
            if(gcd(i, j) == 1)
            {

                cout << lung << "\n";
                return 0;
            }
        }
    }
    cout << -1 << "\n";
}