#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<queue>
#include<map>
#include<cstring>
#include<stdlib.h>
#include<vector>
#include<stack>
#include<math.h>
#define ll long long
#define inf (0x3f3f3f3f)
#define mod (1000000007)
using namespace std;


int main()
{
    int x, y, w, n;
    while (scanf("%d %d %d %d", &x, &y, &w, &n) != EOF)
    {
        int time=0;
        if (x < w)
            time = (n - 1)*(x + y);
        else if (x >= w)
        {
            int turnborn = x / w+1;
            time = (x + y)*((n-1) / turnborn) + w*(n % turnborn?n%turnborn-1:turnborn-1);
        }
        printf("%d\n", time);
    }
    return 0;
}