#include <bits/stdc++.h>
using namespace std;

int main(){
    double a, l, x;
    while(scanf("%lf%lf%lf", &a, &l, &x) != EOF){
        double s_abc = a * sqrt(l * l - a * a / 4) / 2;
        double s_acd = l * sqrt(2 * l * x + x * x) / 4;
        printf("%.12f\n", s_abc + 2 * s_acd);
    }
    return 0;
}