#include <bits/stdc++.h>
using namespace ::std;
int main() {
  long long int temp, q, v1, v2, t, d, i, sum = 0;
  cin >> v1 >> v2 >> t >> d;
  long long int a[t];
  sum = d * (t - 1);
  i = 0;
  temp = 0;
  bool ff = false;
  while (1) {
    if (!ff) {
      i += v1;
      a[temp] = i;
      temp++;
      ff = true;
      continue;
    } else if (i + d - sum + d <= v2) {
      i += d;
      sum -= d;
      a[temp] = i;
    } else {
      for (long long int k = d - 1; k >= -d; k--) {
        if (i + k - sum + d <= v2) {
          i += k;
          sum -= d;
          a[temp] = i;
          break;
        }
      }
    }
    temp++;
    if (temp == t) break;
  }
  long long int s = 0;
  for (long long int j = 0; j < t; j++) s += a[j];
  cout << s;
}
