#include <stdio.h>
#include <math.h>

int main(void) {
	int xa, ya;
	int xb, yb;
	scanf("%d%d", &xa, &ya);
	scanf("%d%d", &xb, &yb);
	xb = -xb;
	if(xa - xb == 0){
		printf("%f\n", (ya + yb) / 2.0);
	}
	else{
		printf("%f\n", (double)abs(xa) * yb / abs(xa - xb) + (double)ya * abs(xb) / abs(xa - xb));
	}

	return 0;
}