#include<iostream>
using namespace std;
int main()
{
    long a,b,r,h,k;
    cin>>a>>b;
    if(a>b)
    {
        r=a;
        a=b;
        b=r;
    }
    h=a;
    k=b;
    while(k!=0)
    {
        r=h%k;
        h=k;
        k=r;
    }
    if(h==1)
        cout<<b-a+1;
    else
        cout<<-1;
}