$S_1:$  The symmetrical difference of sets A and B is  $A\Delta B=(A\cup B)-(A\cap B)$ .
 $S_2 : n$   (set of elements which are in exactly one of the sets  $A,B,C$)   

$=n\left(A\right)-n\left(B\right)+n\left(C\right)-2n(A\cap B)-2n(B\cap C)-2n(A\cap C)+3n(A\cap B\cap C)$
The number of statements which are true is: