#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int vs, ve;
  int t, d;
  cin >> vs >> ve >> t >> d;
  int cur = vs;
  int sm = vs;
  int pre = vs;
  for (int i = 2; i <= t; i++) {
    while (cur < ve && abs(pre - cur) < d) cur++;
    if (cur >= ve) {
      while ((t - i) * d > cur - ve && abs(pre - cur) < d) cur++;
      while ((t - i) * d < cur - ve && abs(pre - cur) < d) cur--;
    }
    pre = cur;
    sm += cur;
  }
  assert(cur == ve);
  cout << sm;
  return 0;
}
