DemoCSharp002

using System;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            double x, y, z;
            bool isCorrect;
            Console.WriteLine( "Задайте x");
            string line = Console.ReadLine();
            if (!double.TryParse(line,out x))
                return;
            Console.WriteLine( "Задайте y");
            line = Console.ReadLine();
            if (!double.TryParse(line, out y))
                return;
            Console.WriteLine( "Задайте z");
            line = Console.ReadLine();
            if (!double.TryParse(line, out z))
                return;
            Console.WriteLine( Geron(x,y,z) );
        }

        static double Geron(double a, double b, double c)
        {
            double p = (a + b + c)/2.0;
            return Math.Sqrt(p*(p - a)*(p - b)*(p - c));
        }
    }
}