try:

=SUMPRODUCT(--ISNUMBER(1/(MATCH($A$2:$A$18&$F$3,$A$2:$A$18&$B$2:$B$18,0)=(ROW($A$2:$A$18)-ROW($A$2)+1))))
or

=COUNT(INDEX(1/(MATCH($A$2:$A$18&$F$3,$A$2:$A$18&$B$2:$B$18,0)=(ROW($A$2:$A$18)-ROW($A$2)+1)),0))