#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main(){
    int a,l,x;
    while(cin >> a >> l >> x){
        double h = sqrt(l*l -a*a/4.0);
        double y = sqrt((l+x)*(l+x) -l*l) /2.0;
        cout << fixed << setprecision(10);
        cout << a*h/2 +l*y << endl;
    }
    return 0;
}
