added assert, input has to be 0 or greater

Aufgabe3/ConsoleApplication1/Program.cs

 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Net.Mime;
 using System.Runtime.CompilerServices;
 using System.Runtime.Remoting.Messaging;
@@ -30,6 +31,8 @@ namespace ConsoleApplication1
                 Environment.Exit(1);
             }
             
+            Debug.Assert(nrProd > -1 && nrCons > -1);
+