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

int main() {
  double a,l,x;
  while(cin >> a >> l >> x) {
    double b=sqrt(l*l-(a/2)*(a/2));
    double c=(l+x)/2;
    double d=sqrt(c*c-(l/2)*(l/2));
    printf("%.10f\n",a*b/2+l*d);
  }
  return 0;
}
