#include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mp make_pair
#define it ::iterator
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double ESP=1e-10;
using namespace std;
int main(){
  double a,l,x;
  while(cin>>a>>l>>x){
    double ans=0;
    double kaku=acos((l*l*2-a*a)/(2*l*l));
    ans+=l*l*sin(kaku)/2;
    double ll=l+x;
    ll/=2;
    double high=sqrt(ll*ll-l*l/4);
    ans+=l*high;
    printf("%.10f\n",ans);
  }

}