#include <bits/stdc++.h>
using namespace std;
int dp[105][2005];
int main() {
  memset(dp, -1, sizeof(dp));
  int v1, v2, t, d;
  cin >> v1 >> v2 >> t >> d;
  dp[1][v1] = v1;
  for (int i = 1; i < t; i++)
    for (int j = 0; j < 2005; j++)
      if (dp[i][j] != -1)
        for (int k = -d; k <= d; k++)
          if (j + k >= 0)
            dp[i + 1][j + k] = max(dp[i + 1][j + k], dp[i][j] + j + k);
  if (dp[t][v2] == -1)
    cout << "0\n";
  else
    cout << dp[t][v2] << '\n';
  return 0;
}
