#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define int long long
#define ALL(x) (x).begin(), (x).end()
#define MAX(x) *max_element(ALL(x))
#define MIN(x) *min_element(ALL(x))
typedef pair<int, int> PI;
typedef pair<int, pair<int, int>> PII;
static const int INF = 1010000000000000017LL;
static const double eps = 1e-12;
static const double pi = 3.14159265358979323846;
static const int dx[4] = {1, -1, 0, 0};
static const int dy[4] = {0, 0, 1, -1};
static const int ddx[8] = {1, -1, 0, 0, 1, 1, -1, -1};
static const int ddy[8] = {0, 0, 1, -1, 1, -1, 1, -1};
template <class T>
inline bool chmin(T& a, T b) {
if (a > b) {
a = b;
return true;
}
return false;
}
template <class T>
inline bool chmax(T& a, T b) {
if (a < b) {
a = b;
return true;
}
return false;
}
int S, A, B, X;
signed main() {
cin >> S >> A >> B >> X;
int m = X / (A + B);
int x = X - m * (A + B);
int ans = m * S * A;
if (x <= A) {
ans += S * x;
} else {
ans += S * A;
}
cout << ans << endl;
}