#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
double a,l,x;
int main(){
  while(cin>>a>>l>>x){
    double b=sqrt(l*l-0.25*a*a);
    double h=sqrt((2.0*l*x+x*x)*0.25);
    printf("%.8f\n",a*b*0.5+l*h);
  }
  return 0;
}