#include <iostream>
using namespace std;
int main()
{
    int n, v, m, a;
    int num = 0;
    cin >> n >> v >> m >> a ;
    for (int m = 1; m <= n; m++) {
      if (m == 1) {
            num = v;
        }
        else {
            v = v + a;
            num = num + v;
        }
    }
    cout << num << endl;
    return 0;
}