#include <iostream>
using namespace std;


long long a,b,x,y,mx,aux,i,j;

long long cmmdc(long long a,long long b)
{
    long long r=1;
    while(r!=0)
    {
        r=a%b;
        a=b;
        b=r;
    }
    return a;
}

int main()
{
    cin>>a>>b;
    x=a;
    while (cmmdc(x,b)!=1 && x<=b)
        x++;
    y=b;
    while (cmmdc(y,b)!=1 && y>=a)
        y--;
    mx=max(b-x+1,y-a+1);
    if (mx<=0)
    {
        cout<<-1;
        return 0;
    }
    if (x>y)
    {
        aux=x;
        x=y;
        y=aux;
    }
    for (i=a;i<=x;i++)
        for (j=i+mx;j<=b;j++)
        {
            if (cmmdc(i,j)==1 && j-i+1>mx)
                mx=j-i+1;
        }
    cout<<mx;

    return 0;
}