#include <iostream>

using namespace std;

int sum1[25] = { 0, 0, 1, 2, 2, 3, 4, 6, 9, 14, 22, 35, 56, 90, 145, 234, 378, 611, 988, 1598, 2585, 4182, 6766, 10947 }, sum2[25] = { 0, 0, 0, 0, 1, 2, 4, 7, 12, 20, 33, 54, 88, 143, 232, 376, 609, 986, 1596, 2583, 4180, 6764, 10945, 17710 };

int main() {
  int a, n, m, x;
  cin >> a >> n >> m >> x;
  cout << a * sum1[x] + (m - a * sum1[n - 1]) / sum2[n - 1] * sum2[x];
  return 0;
}