#include <bits/stdc++.h>
int main() {
  double a, b, c, s, t;
  while (~scanf("%lf%lf%lf", &a, &b, &c)) {
    s = sqrt(b * b - a * a / 4) * a / 2;
    t = sqrt(c * c + 2 * b * c) * b / 2;
    printf("%lf\n", s + t);
  }
}
