/*
  AOJ 1091
  Title:KND is So Sexy
  @kankichi573
*/
#include <stdio.h>
#include <math.h>
#include <float.h>

double a,l,x;
double h1,h2,S;
extern double sqrt(double);
main()
{
  while(EOF!=scanf("%lf %lf %lf",&a,&l,&x))
    {
      h1=sqrt(l*l-a*a/4.0);
      S=a*h1/2;
      h2=sqrt((l+x)*(l+x)/4.0-l*l/4.0);
      S += h2*l;
      printf("%.7lf\n",S);
    }
  return(0);
}