#include <iostream>

using namespace std;

int main()
{
    long long int a, b, i, y, mx=-1, m, n, r;
    cin>>a>>b;
    for(i=a; i<b; i++)
        for(y=i+1; y<=b; y++)
        {
            m=i;
            n=y;
            r=m%n;
            if(i%2!=0 || y%2!=0)
                while(r!=0)
                {
                    m=n;
                    n=r;
                    r=m%n;
                }
            if(n==1 && mx<y-i+1) mx=y-i+1;
        }
    cout<<mx;

    return 0;
}