#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <cmath>
#include <queue>
#include <limits>
#include <iomanip>

using namespace std;

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int maxb = d / (c + 1);
    int ans = min(a, maxb); 
    ans = min(c * b, ans); 
    cout << ans << endl;
}