#include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;
double heron(double a,double b,double c){
  double s = (a+b+c)/2;
  return sqrt(s*(s - a)*(s - b)*(s - c));
}
int main(){
  double a,l,x;
  while( cin >> a >> l >> x ){
    double m,s;
    m = (x+l)/2;
  
    s = 2*heron(m,m,l) + heron(a,l,l);
    printf("%.7f\n",s);
} 
}