/************************************************

* Author        :Running_Time

* Created Time  :2015-8-22 18:55:05

* File Name     :A.cpp

 ************************************************/

#include <cstdio>

#include <algorithm>

#include <iostream>

#include <sstream>

#include <cstring>

#include <cmath>

#include <string>

#include <vector>

#include <queue>

#include <deque>

#include <stack>

#include <list>

#include <map>

#include <set>

#include <bitset>

#include <cstdlib>

#include <ctime>

using namespace std;

#define lson l, mid, rt << 1

#define rson mid + 1, r, rt << 1 | 1

typedef long long ll;

const int N = 1e5 + 10;

const int INF = 0x3f3f3f3f;

const int MOD = 1e9 + 7;

int main(void)    {

    int x, y, w, n;

    while (scanf ("%d%d%d%d", &x, &y, &w, &n) == 4)	{

        int cnt = 0;

        int t = 0;

        int a = x / w + 1;

        while (cnt + a <= n)	{

            cnt += a;

            if (cnt == n)	{

                t += (a - 1) * w;	break;

            }

            else if (cnt == n - 1)	{

                t += x + y;	break;

            }

			else t += (x + y);

        }

        if (cnt == n || cnt == n - 1)	{

            printf ("%d\n", t);	continue;

        }

        cnt++;		//忘写，WA一次

        while (cnt < n)	{

            t += w;

            cnt++;

        }

        printf ("%d\n", t);

    }

    return 0;

}