Input string was not in a correct format.

Hi, I just started learning asp.net
i set simple application but i get this error that  "Input string was not in a correct format"
Input string was not in a correct format. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.FormatException: Input string was not in a correct format.

Source Error:

Line 10: protected void Page_Load(object sender, EventArgs e) Line 11: { Line 12: decimal num1 = Decimal.Parse(txtNumber1.Text); Line 13: decimal num2 = Decimal.Parse(txtNumber2.Text); Line 14: decimal ans = num1 + num2; ...