#include <iostream>

using namespace std;

int main(){

	int a, b, c, d, s1 = 0, s2 = 0;

	cin>>a>>b>>c>>d;

	for(int n = 1;n < 10;n++)
	{
		s1 = b+(2*a+(n-1)*a)*n/2;
		s2 = d+(2*c+(n-1)*c)*n/2;

		if(s1 == s2){
			cout<<s1;
			return 0;
		}
	}
	cout<<-1;

}