#include<bits/stdc++.h>
#include<math.h>
using namespace std;
int main(void){
    double a,l,x;
    double h,s;
    while(cin>>a>>l>>x){
        s=sqrt(a*a*(double)l*(double)l/4.0-a*a*a*a/16.0);
        h=sqrt(pow((l+x)/2.0,2)-l*l/4.0);
        s+=h*l;
        printf("%.10f\n",s);
    }
}