#include<cstdio>
#include<vector>
#include<algorithm>
#include<iostream>
#include<cmath>
using namespace std;
double h(double a,double b,double c){
    double s=(a+b+c)*0.5;
    return sqrt(s*(s-a)*(s-b)*(s-c));
}
int main(){
    double a,l,x;
    while(cin>>a>>l>>x)
        cout<<fixed<<h(a,l,l)+h(l,(l+x)*0.5,(l+x)*0.5)*2<<endl;


    return 0;
}