My data contains 2 columns of data

I want to count column A if column B has not already been counted.
A B
1951 X911
2614 X911
2618 X911
2614 X911
1031 X911
2614 X911
2614 WAT
2614 WAT
2614 WAT
2614 WAT
2614 WAT

In other words, I want to see how many different lines of X911. The answer
I want is 3 because there are 3 different numbers associated with X911

HELP!!!!