#include <stdio.h>
#include <math.h>
int main(){

  double a,l,x,hon,yoko;

  while(scanf("%lf %lf %lf",&a,&l,&x) != EOF){

  hon=sqrt(l*l-a*a/4)*a/2;

  yoko=sqrt((l*l+2*l*x+x*x)/4-l*l/4)*l;

  printf("%f\n",hon+yoko);
  
  }

  return 0;

}