import static java.lang.Math.sqrt;

import java.io.File;
import java.io.PrintStream;
import java.util.Scanner;

public class StainedCarpet_constant1_artur {
  public static void main(String[] args) throws Exception {
    Scanner in = new Scanner( System.in );
    PrintStream out = System.out;

    double pa = in.nextDouble(), pb = in.nextDouble(), pc = in.nextDouble();
    double result = -1;
    if (!(pb + pc < pa || pa + pc < pb || pa + pb < pc)) {
      double p = (pa + pb + pc) / 2;
      //System.out.println(p + " " + pa + " " + pb + " " + pc);
      double s = sqrt(p * (p - pa) * (p - pb) * (p - pc));
      double t = pa * pa + pb * pb + pc * pc;
      result = (sqrt(3) / 4 * t + 3 * s) / 2;

      double side = Math.sqrt(result * 4 / Math.sqrt(3));
      result = side < pa || side < pb || side < pc ? -1 : result;
    }
    out.printf("%.3f\n", result);
    out.close();
  }
}