#include <iostream>
#include <vector>
using namespace std;
int64_t gdc(int64_t a,int64_t b)
{
     return b == 0 ? a : gdc(b, a % b);
}
int main()
{
int64_t a,b;
cin>>a>>b;
if(gdc(a,b)==1)
 {

 cout<<b-a+1;
 return 0;
}
    int64_t t=b;
while(1)
{
    b--;
    if(gdc(a,b)==1)
    {
        cout<<b-a+1;
        return 0;
    }
    if(b==0)
        break;
}
while(1)
{
    a++;
    if(gdc(a,b)==1LL)
    {
        cout<<b-a+1;
         return 0;
    }
    if(a==b)
        break;
}
cout<<"-1";
}