I am getting error like this "System.FormatException was unhandled " Help me

hello friends

I can not convert string to double , I am new to C sharp . Please help me out
I tryed parse,covert, everything . I have pasted the code below .

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class sales
    {
        double commrate = 0.01;
        String andrea;
        String emily;
        String tom;
        double andreaAmount;
        double emilyAmount;
        double tomAmount;
        double amount;
	void getSalesName()
	{
        Console.WriteLine("Enter the sale person name intial and If u want to exit press f");
		String s = Console.ReadLine();
	     
	     if(s.Equals("A")|| s.Equals("B")|| s.Equals("T"))
	     {
	    	 Console.WriteLine("Enter the amount of sales");
             Console.ReadLine();
             string s1 = Console.ReadLine();
               // Error in the below line 
	    	  amount = double.Parse(s1);
                         
			 if(s.Equals("A"))
	    	 {
	    		 andreaAmount = andreaAmount + amount *commrate ; 
	     	 }
	    	 else if(s.Equals("B"))
	    	 {
	    		 emilyAmount = emilyAmount + amount *commrate;
                     	 }
	    	 else
	    	 {
	    		tomAmount = tomAmount + amount * commrate ; 
	    	 }
	    	 
			 getSalesName();
	     }
	     else if(!s.Equals("F"))
	     {
	    	 Console.WriteLine("please reEnter the corrent Intial of the salesPersonName");
             Console.Read();
	    	 getSalesName();
	     }
	     else if(s.Equals("F")) 
	     {
	    	 
	    	 Console.WriteLine("The total sales person sales are");
	    	 Console.Write("Andrea " + andreaAmount);
	    	 Console.Write("Emily " + emilyAmount);
             Console.Write("Tom " + tomAmount);
             Console.ReadLine();  		   
	     }
		
	}

        static void Main(string[] args)
        {
            sales sa = new sales();
            sa.getSalesName();
        }
    }
}