#include
using namespace std;
int main() {
    double a,b,c,d,r,h;
    cin >> a >> b >> c;
    d = (b - a) / 2;
    h = sqrt(c*c - d*d);
    r = h/4;
    cout << setprecision(2) << fixed << (a + b)/2*h - r*r*3.14;
    return 0;
}

#include
#include
#include
using namespace std;

int main() {
    double a, b, c;
    cin >> a >> b >> c;
    double h = sqrt(c * c - pow((b - a) / 2, 2));
    double S = (a + b) * h / 2;
    double r = h / 4;
    double S_gieng = 3.14 * r * r;
    double S_con_lai = S - S_gieng;
    cout << fixed << setprecision(2) << S_con_lai << endl;
    return 0;
}