import java.util.Scanner;

class Main{
	public static void main(String[] args) {
		new Main().run();
	}
	
	void run() {
		Scanner sc=new Scanner(System.in);
		int A=sc.nextInt();
		int B=sc.nextInt();
		if(gcd(A,B)!=1) {
			System.out.println(-1);
		}else {
			boolean[] ok=new boolean[100000];
			for(int x=0;x<=2*B;++x) {
				for(int y=0;y<=2*A;++y) {
					ok[A*x+B*y]=true;
				}
			}
			int ans=0;
			for(int i=0;i<=2*A*B;++i) {
				if(!ok[i])++ans;
			}
			System.out.println(ans);
		}
		
	}
	
	int gcd(int a,int b) {
		return a==0?b:gcd(b%a,a);
	}
}