'Rextester.Program.Main is the entry point for your code. Don't change it.
'Compiler version 11.0.50709.17929 for Microsoft (R) .NET Framework 4.5

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text.RegularExpressions

Namespace Rextester
    Public Module Program
        Public Sub Main(args() As string)
        
            Dim score As Integer
            
            'get user input
            Console.Writeline("\n\tEnter score: ")
            score= Console.ReadLine()
            
            'show grade
            Console.WriteLine("\n\tYour grade: " & calcGrade(score) )
        End Sub
        
        'convert number score to letter grade
        Function calcGrade(ByVal sco As Double) As String
            Dim gra As String

            'detect invalid score
            If sco<0 Or sco>100 Then
                gra= "invalid"
                return gra
            End If

            Select Case sco
                Case >=90
                    gra= "A"
                Case >=80
                    gra= "B"
                Case >=70
                    gra= "C"
                Case >=60
                    gra= "D"
                Case Else
                    gra= "F"
            End Select

            return gra
        End Function 
        
        
    End Module
End Namespace