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

int main()
{
	int C,r1,r2;
	scanf ("%d %d %d",&C,&r1,&r2);

	double c = (r1 + r2) / 2.;
	double r = (r2 - r1) / 2.;
	double ans = 2 * C * acos(-1.0) * acos(-1.0) * c * r * r;
	printf ("%.12lf\n",ans);

	return 0;
}