Skip blanks and copy data from other worksheets

Hi All,

I have 4 worksheets named "Main","Kevin","chris" and "Alan"
Only some cells in the B column are updated in the "Kevin","Chris" and "Alan" worksheets and some are blank. Please find attached the sample file for ur refernce.

Could anyone help me by providing a macro to copy the data from B column from the 3 worksheets and paste it in the B column of "Main" excluding the blank cells . If any cell in the column B is blank in the other sheet, the "Main" worksheet also should be blank...

Thanks in advance for spending ur time in it..