#include <bits/stdc++.h>
using namespace std;
long double a, l, x, p;
int main() {
  while (cin >> a >> l >> x) {
    long double z = sqrt(l * l - a * a / 4);
    p = 0;
    p += z * a / 2;
    z = sqrt((l + x) * (l + x) / 4 - l * l / 4);
    p += z * l;
    printf("%.15f\n", p);
  }
  return 0;
}
