package yukicoder;

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader read = new BufferedReader(new InputStreamReader(System.in));

		try {
			String[] box = read.readLine().split(" ");
			double[] num = new double[4];
			for (int i = 0; i < 4; ++i) {
				num[i] = Double.parseDouble(box[i]);
			}

			double ans = 0;
			if (num[2] * num[1] / num[0] <= num[3]) {
				ans = num[2] * num[1] / num[0] + num[2];
			} else {
				ans = num[3] * num[0] / num[1] + num[3];
			}

			System.out.println(ans);

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}