planta

#include <fstream>
using namespace std;

int main()
{
    unsigned long long int d,a,b,n;
    ifstream f("planta.in");
    f>>d>>a>>b>>n;
    f.close();
    unsigned long long int h=d+(a-b)*(n/2);
    if(n%2==1)
        h+=a;
    ofstream g("planta.out");
    g<<h<<'\n';
    g.close();
    return 0;
}