$a_{1,1}$ : number of samples misclassified by both $f_a$ and $f_b$	$a_{1,2}$ : number of samples misclassified by $f_a$ but not $f_b$
$a_{2,1}$ : number of samples misclassified by $f_b$ but not $f_a$	$a_{2,2}$ : number of samples misclassified by neither $f_a$ nor $f_b$