#include <bits/stdc++.h>
using namespace std;
int binarySearch(int arr[], int l, int r, int x) {
  while (l <= r) {
    int m = l + (r - l) / 2;
    if (arr[m] == x) return m;
    if (arr[m] < x)
      l = m + 1;
    else
      r = m - 1;
  }
  return -1;
}
bool isprime(int n) {
  if (n <= 1) return false;
  for (int i = 2; i < n; i++)
    if (n % i == 0) return false;
  return true;
}
long long int factorial(int n) {
  if (n == 0) return 1;
  return n * factorial(n - 1);
}
int countDigit(long long n) {
  int count = 0;
  while (n != 0) {
    n = n / 10;
    ++count;
  }
  return count;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int v1, v2, t, d, sum = 0;
  cin >> v1 >> v2 >> t >> d;
  for (int i = 0; i < t; i++) sum += min(v2 + (t - 1 - i) * d, v1 + i * d);
  cout << sum;
  return 0;
}
