#include <iostream>
#include <climits>

using namespace std;

FILE *f,*g;

int main ()
{
    int a,b,j,i,r,x,y,dif,max;
    cin>>a>>b;
    max=INT_MIN;
    if (a==1)
        cout<<b;
    else
    {

        for (i=a; i<b; i++)
        {
            for (j=b; j>i; j--)
            {
                x=j;
                y=i;
                r=1;
                while (x%y)
                {
                    r=x%y;
                    x=y;
                    y=r;
                }
                if (r==1)
                    dif=j-i+1;
                if (dif>max)
                    max=dif;
            }
        }
        if (max==INT_MIN)
            cout<<-1;
        else
            cout<<max;
    }
}