#include <fstream>

using namespace std;

ifstream fin("planta.in");
ofstream fout("planta.out");

int main() {
	int d,a,b,n,i;
	fin >> d >> a >> b >> n;
	int z = n / 2;
	d = d + a * z - b * z;
	if (n % 2 == 1) {
		d = d + a;
	}
	fout << d;
	return 0;
}