# include <stdio.h>

int main()
{
	double x, a, y, b, z;
	printf("请输入四个数：\n");
	scanf("%lf %lf %lf %lf", &x, &a, &y, &b);
	z = (a * x - b * y) / (a - b);
	printf("%.2lf", z);
	return 0;
}