import java.util.Scanner;

public class PolygonArea_TernarySearchACC{
	
	static double Area(double k1, double k2, double k3) // What is the area of a triangle with sides k1, k2, k3?
	{
		// Heron.
		double s = (k1 + k2 + k3) / 2.0;
		return Math.sqrt(s * (s - k1) * (s - k2) * (s - k3));
	}
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		double[] l = new double[4];
		
		for(int i = 0; i < 4; i++)
			l[i] = sc.nextDouble();
		
		// Assume the starting point is in the origin.
		// By rotating the shape, we can assume that the point opposite to the starting point is on the positive x-axis.
		// Now do binary search on this point opposite the starting point, using only some elementary geometry.
		// By splitting the tetragon into two triangles, calculations become much simpler.
		double mink = Math.max(Math.abs(l[0] - l[1]), Math.abs(l[2] - l[3]));
		double maxk = Math.min(l[0] + l[1], l[2] + l[3]);
		
		double answer = 0;
		
		while(maxk - mink > 0.0000001)
		{			
			double k1 = (mink*2 + maxk) / 3.0d;
			double k2 = (mink + maxk*2) / 3.0d;
			
			double answer1 = Area(k1, l[0], l[1]) + Area(k1, l[2], l[3]);
			double answer2 = Area(k2, l[0], l[1]) + Area(k2, l[2], l[3]);
			
			if(answer1 >= answer2) maxk = k2;
			if(answer2 > answer1) mink = k1;
			
			answer = answer1;
		}
		
		System.out.println(answer);
		
	}
}