#include <iostream>
#include <cmath>
#include <cstdio>

using namespace std;
typedef double dou;
dou area(dou a, dou b, dou c){
    dou s = (a+b+c)/2;
    return sqrt(s*(s-a)*(s-b)*(s-c));
}

int main(){
    dou a,l,x;
    while(cin >> a >> l >> x){
        dou s1 = area(l,l,a);
        dou t = (l+x)/2;
        dou s2 = area(t,t,l);
        printf("%.10f\n",s1 + 2*s2);
    }
}