definiens: shall mean, with respect to any TYSABRI Transaction : ( A)if the aggregate Net Sales for the calendar year ending immediately prior to the consummationof such TYSABRI Transaction was less than or equal to two billion dollars ( $ 2,000,000,000),eighteen percent ( 18 % ) ; or ( B)if the aggregate Net Sales for the calendar year ending immediately prior to the consummationof such TYSABRI Transaction was more than two billion dollars ( $ 2,000,000,000 ), the amount(expressed as a percentage ) obtained by dividing ( x ) the sum of three hundred and sixty million($360,000,000 ) and twenty - five percent ( 25 % ) of the amount by which the aggregate Net Salesfor the 45 calendar year ending immediately prior to the consummation of such TYSABRI Transactionexceeds two billion dollars ( $ 2,000,000,000 ) by ( y ) the aggregate Net Sales for the calendar yearending immediately prior to such TYSABRI Transaction