#include <iostream>
#include <cstdio>
#include <math.h>
using namespace std;

int main(){
  int a,l,x;
  while( cin >> a >> l >> x ){
    double result = a * sqrt( l*l - a * a / 4.0 ) / 2.0 +
      ( l * sqrt( ( (l+x) * (l+x) / 4.0 ) - l * l/4.0 )  ) ;
    printf("%.10f\n",result);
  }

  return 0;
}