#include<stdio.h>
#include<math.h>
double f(int a,double b){
  double s=(a+b+b)/2;
  return sqrt(s*(s-a)*(s-b)*(s-b));
}
int main(){
  int a,l,x;
  double s;
  while(scanf("%d %d %d",&a,&l,&x)!=EOF){
    s =f(a,(double)l);
    s+=f(l,(l+x)/2.0)*2;
    printf("%lf\n",s);
  }
  return 0;
}