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

using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)

int a, l, x;

double s(double b, double c) 
{
    double h = sqrt(c*c-b*b/4);
    return b*h/2;
}


void solve()
{
    double ans = s(a, l)+2*s(l, (l+x)/2.0);
    printf("%.6lf\n", ans);
}

int main()
{
    while (cin >> a >> l >> x) solve();
    return 0;
}
