import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;

public class main {
	public static void main(String[] args) throws FileNotFoundException {
		Scanner in = new Scanner(new File("input.txt"));
		int m = in.nextInt(), n = in.nextInt();
		float t = (float) in.nextInt(), c = (float) in.nextInt();
		in.close();
		double p1 = (t-c)*(t-c)*100/(t*t);
		double p2 = c*c*(m-2)*(n-2)*Math.PI/(4*m*n*t*t);
		PrintWriter out = new PrintWriter(new File("output.txt"));
		System.out.printf("%.4f",p1);
		out.flush();
		out.close();
	}
}
