#include <iostream>

using namespace std;
int gcd(int x, int y){
	if(y>x) swap(x,y);
	if(y==0) return x;
	return gcd(x%y,y);
}
int c[20010];
int main(){
	int i,j,a,b,ans = 0;
	cin >> a >> b;
	if(gcd(a,b)!=1){
		cout << -1 << endl;
		return 0;
	}
	for(i=1;i<=a*b;i++){
		c[i] = 1;
	}
	for(i=0;i<=b;i++){
		for(j=0;j<=a;j++){
			c[a*i + b*j] = 0;
		}
	}
	for(i=1;i<=a*b;i++){
		ans += c[i];
	}
	cout << ans << endl;
}