Creating assembly and adding strong name but it is not adding

Question

Hi i am creating assembly and adding strong key in this way but it is not adding can any one tell me where i am getting mistake..

:\c-sharp>csc /t:library add.cs

Microsoft (R) Visual C# 2010 Compiler version 4.0.30319.1
Copyright (C) Microsoft Corporation. All rights reserved.

D:\c-sharp>sn -k mykey.snk

Microsoft (R) .NET Framework Strong Name Utility Version 4.0.30319.1
Copyright (c) Microsoft Corporation. All rights reserved.

Key pair written to mykey.snk

D:\c-sharp>sn -Vr add.dll

Microsoft (R) .NET Framework Strong Name Utility Version 4.0.30319.1
Copyright (c) Microsoft Corporation. All rights reserved.

add.dll does not represent a strongly named assembly

D:\c-sharp>sn -R add.dll mykey.snk

Microsoft (R) .NET Framework Strong Name Utility Version 4.0.30319.1
Copyright (c) Microsoft Corporation. All rights reserved.

add.dll does not represent a strongly named assembly

D:\c-sharp>