#include <cmath>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <ctime>
#include <set>
#include <map>
#include <list>
#include <queue>
#include <string>
#include <vector>
#include <fstream>
#include <iterator>
#include <iostream>
#include <algorithm>
using namespace std;
#define LL long long
#define INF 0x3f3f3f3f
#define MOD 1000000007
#define eps 1e-6
#define MAXN 1000000
#define MAXM 100
#define dd cout<<"debug"<<endl
#define pa {system("pause");}
#define p(x) printf("%d\n", x)
#define pd(x) printf("%.7lf\n", x)
#define k(x) printf("Case %d: ", ++x)
#define s(x) scanf("%d", &x)
#define sd(x) scanf("%lf", &x)
#define mes(x, d) memset(x, d, sizeof(x))
#define do(i, x) for(i = 0; i < x; i ++)
#define dod(i, x, l) for(i = x; i >= l; i --)
#define doe(i, x) for(i = 1; i <= x; i ++)
int x, y, w, n;

int main()
{
    while(scanf("%d %d %d %d", &x, &y, &w, &n) != EOF)
    {
        int ff = x / w + 1;
        int ans;
        int temp_b = (n - 1) / ff;
        int temp_a = (n - 1) % ff;
        if(!temp_a) ans = temp_b * (x + y);
        else 
        {
            ans = temp_b * (x + y) + temp_a * w;
        }
        printf("%d\n", ans);
    }
    return 0;
}