#include <iostream>
using namespace std;

int main()
{
	freopen("dragon.in", "r", stdin);
	freopen("dragon.out", "w", stdout);
	int s, k, a, b, legs;
	cin>>s>>k>>a>>b;
	legs=b-(a-k)*s;
	if(legs<0 || a-k<0)
	{
		cout<<-1;
		return 0;
	}
	cout<<legs;
	return 0;
}