#include<iostream>
using namespace std;
int A,B;
int gcd(int a,int b){return b?gcd(b,a%b):a;}
int ex[10101];
main()
{
	cin>>A>>B;
	if(gcd(A,B)>1)
	{
		cout<<-1<<endl;
	}
	else
	{
		int cnt=A*B;
		for(int x=0;x<=B;x++)for(int y=0;y<=A;y++)
		{
			int now=x*A+y*B;
			if(1<=now&&now<=A*B)
			{
				cnt-=!ex[now]++;
			}
		}
		cout<<cnt<<endl;
	}
}