#include <bits/stdc++.h>
using namespace std;
int main( int argc, char *argv[] )
{int a,b,c;
c=1;
cin>>a>>b;
while(c<=10^18&&((c==a)||(c==b)||(c+a)%b==0||(c+b)%a==0||(a+b)%c==0))
{c=c+1;}
if(c==10^18+1){cout<<-1<<endl;}
else{cout<<c+1<<endl;}

	return 0;
}