making pairs and subtracting the elements making the pair

23 buy

45 buy

67 buy

96 buy

110 buy

77 sell

89 sell

210 sell

456 sell

900 sell

on left is the row values of an excel sheet. the above buy and sell are strings stored in a column of the excel sheet.THESE BUY AND SELL HAVE NUMERICAL VALUES ASSOCIATED WITH THEM IN ANOTHER COLUMN.I WANT TO SUBTRACT NUMERICAL VALUE ASSOCIATED WITH FIRST BUY FROM THE FIRST SELL IN THE BUY/SELL COLUMN AND THEN SECOND BUY VALUE FROM SECOND SELL VALUE AND SO ON.HOW TO DO?