#!/usr/local/bin/tcc -run
#include <stdio.h>
#include <math.h>

int main() {
	/*double a,b,c,d;

	scanf_s("%lf", &c);

	a = pow((1.0 - (c / 2.0)), 2.0);

	b = 1.0 / (sqrt(a + (1.0 / 4.0)));

	printf("%lf\n", a);
	printf("%lf\n\n", b);


	d = 1.0 / (sqrt((pow((1.0 - (c / 2.0)), 2.0)) + (1.0 / 4.0)));


	printf("%lf", d);
	*/

	double x, y = 0;

	scanf("%lf", &x);

	y = 1.0 / (sqrt((pow((1.0 - (x / 2.0)), 2.0)) + (1.0 / 4.0)));

	printf("%lf", y);
}