#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T bigmod(T p, T e, T M) {
  long long ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return (T)ret;
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
inline T modinverse(T a, T M) {
  return bigmod(a, M - 2, M);
}
template <class T>
inline T lcm(T a, T b) {
  a = abs(a);
  b = abs(b);
  return (a / gcd(a, b)) * b;
}
template <class T, class X>
inline bool getbit(T a, X i) {
  T t = 1;
  return ((a & (t << i)) > 0);
}
template <class T, class X>
inline T setbit(T a, X i) {
  T t = 1;
  return (a | (t << i));
}
template <class T, class X>
inline T resetbit(T a, X i) {
  T t = 1;
  return (a & (~(t << i)));
}
inline long long power(long long a, long long b) {
  long long multiply = 1;
  for (int i = (0); i < (b); i++) {
    multiply *= a;
  }
  return multiply;
}
int v2, d;
long long dp[100005][105];
long long calc(int now, int t) {
  if (now <= 0) {
    return -200000000000000000LL;
  }
  if (t == 0) {
    if (v2 == now) return (long long)now;
    return -200000000000000000LL;
  }
  if (dp[now][t] != -1) return dp[now][t];
  long long ans = 0;
  long long x = -200000000000000000LL;
  for (int i = 0; i <= d; i++) {
    x = max(x, calc(now + i, t - 1));
    x = max(x, calc(now - i, t - 1));
  }
  return dp[now][t] = now + x;
}
int main() {
  ios_base::sync_with_stdio(0);
  int v1, t;
  memset(dp, -1, sizeof(dp));
  cin >> v1 >> v2 >> t >> d;
  cout << calc(v1, t - 1) << endl;
  return 0;
}
