#include<cstdio>
#include<cmath>
using namespace std;

double h(double a,double b,double c){
  double s = (a+b+c)/2;
  return sqrt( (s-a)*(s-b)*(s-c)*s );
}

int main(){
  double a,l,x;
  while(scanf("%lf%lf%lf",&a,&l,&x)!=EOF)
    printf("%.8lf\n",h(l,l,a)+2*h((l+x)/2,(l+x)/2,l));
}