#include<iostream>
using namespace std;
long long a,b;
long long cmmdc(long long A, long long B)
{
    int R;
    while (B)
        R=A%B, A=B, B=R;
    return A;
}
int main()
{
    cin>>a>>b;
    if (a==b && a>1)
    {
        cout<<"-1\n";
        return 0;
    }
    if (a==1)
    {
        cout<<b-a+1<<"\n";
        return 0;
    }
    int i,st=1,dr=b-a,mij;
    while (st<=dr)
    {
        mij=(st+dr)>>1;
        for (i=a;i+mij<=b;++i)
            if (cmmdc(i,i+mij)==1)
                break;
        if (i+mij<=b)
            st=mij+1;
        else
            dr=mij-1;
    }
    cout<<st<<"\n";
    return 0;
}