#include <iostream>
#include <cmath>
#include <stdio.h>
using namespace std;
const int MOD = 1000000007;

int main() {
    int y, w, n, x;
    while (scanf("%d", &x) != EOF) {
        scanf("%d %d %d", &y, &w, &n);
        int cycle = ceil((x+1) / (0.0 + w));
        int cycles = (n - 1) / (cycle);
        int left = n - cycle * cycles;
        int k = (left-1) * w;
        cout << (x+y) * cycles + k << endl;
    }
}