validation in excel sheet !

Question

i have two columns 

type	month
w	3
s	4
a	4

 

type has a dropdown having => w,s,a 

month has a dropdown having => 3,4

3 is marvh and 4 is april

 

i want to validate column month in a way that when i select "a" in type,  in month dropdown only month 4 should be available

"s" should have both 3,4 in dropdown of month column

"w"  should have only 3 in dropdown of month column