diff --git a/Program.cs b/Program.cs
index f946490..df34652 100644
--- a/Program.cs
+++ b/Program.cs
@@ -7,31 +7,58 @@ namespace calculator
   {
     static void Main(string[] args)
     {
+
       if (args.Length == 0)
       {
-        string input = readREPL();
+        string input = REPL.read();
         while (input != null)
         {
-          writeREPL(evaluate(input));
-          input = readREPL();
+          handleExpression(input);
+          input = REPL.read();
         }
       }
       else
-      {
-        writeREPL(evaluate(args[0]));
-      }
+        handleExpression(args[0]);
+
     }
 
-    static private double evaluate(string s)
+    static private void handleExpression(string s)
     {
       try
       {
-        string[] expr = s.Split();
-        Stack<double> st = new Stack<double>();
+        RPN calc = new RPN(s);
+        REPL.write(calc.evaluate());
+      }
+      catch (Exception e)
+      {
+        Console.WriteLine(e.Message);
+      }
+    }
+  }
 
-        for (int i = 0; i < expr.Length; i++)
+  class RPN
+  {
+    Stack<double> st = new Stack<double>();
+    string[] tokens;
+
+    public RPN(string s)
+    {
+      tokens = s.Split();
+    }
+    public RPN(string[] ss)
+    {
+      tokens = new string[ss.Length];
+      ss.CopyTo(tokens, 0);
+    }
+
+    public double evaluate()
+    {
+
+      for (int i = 0; i < tokens.Length; i++)
+      {
+        try
         {
-          object val = convertOne(expr[i]);
+          object val = convertOne(i);
 
           if (val is double number)
             st.Push(number);
@@ -59,36 +86,57 @@ namespace calculator
                   st.Push(Math.Pow(a, b));
                   break;
                 default:
-                  throw new Exception("Got unknown operator");
+                  throw new Exception("got unknown operator");
               }
             }
             catch (InvalidOperationException)
             {
-              throw new Exception(oper + " operation requires two operands");
+              throw new Exception("operator requires 2 arguments");
             }
           }
-          else throw new Exception("Stack corrupted");
         }
+        catch (Exception e)
+        {
+          throw new Exception(positionPointer(i, 2) + ' ' + e.Message);
+        }
+      }
 
-        return st.Pop();
-      }
-      catch (Exception e)
-      {
-        Console.WriteLine(e.Message);
-        Environment.Exit(1);
-        return -1;
-      }
+      if (st.Count != 1) throw new Exception("Expression is not finished");
+
+      return st.Pop();
     }
 
-    static private object convertOne(string s)
+    private string positionPointer(int pos, int gap)
     {
+      string prepend = "";
+      for (int i = 1; i <= gap && pos - i >= 0; i++)
+        prepend = tokens[pos - i] + ' ' + prepend;
+      if (pos > gap) prepend = "... " + prepend;
+
+      string append = " ";
+      for (int i = 1; i <= gap && pos + i < tokens.Length; i++)
+        append += tokens[pos + i] + ' ';
+
+      if (pos < tokens.Length - 2) append += "...";
+
+      string arrow = "^".PadLeft(prepend.Length + 1);
+
+      return prepend + tokens[pos] + append + '\n' + arrow;
+    }
+
+    private object convertOne(int i)
+    {
+      string s = tokens[i];
       double res;
       if (Double.TryParse(s, out res)) return res;
       else if (s.Length == 1) return s[0];
-      else throw new Exception("Got multiple characters operator");
+      else throw new Exception("got multiple characters operator");
     }
+  }
 
-    static private string readREPL()
+  class REPL
+  {
+    static public string read()
     {
       Console.Write(">> ");
       string input = Console.ReadLine();
@@ -107,7 +155,7 @@ namespace calculator
       }
     }
 
-    static private void writeREPL<T>(T o)
+    static public void write<T>(T o)
     {
       Console.WriteLine(o);
     }