#include <iostream>
#include <vector>
using namespace std;
long long gdc(long long A, long long B)
{
	long long c=A%B;
	while(c)
	{
		A=B;
		B=c;
		c=A%B;
	}
	return B;
}
int main()
{
int64_t a,b;
cin>>a>>b;
bool ok=1;
if(gdc(a,b)==1LL)
 {
ok=0;
 cout<<b-a+1;
 return 0;
}


while(a<b && ok)
{
   for(int i=a;i<b && ok;i++)
    if(gdc(i,b)==1LL)
    {
        ok=0;
        cout<<b-i+1;
        return 0;
    }
    b--;
}
if(ok)
cout<<"-1";
}