Truncate expression

Hi, 

I have SSN which is in this format 055-68-6748 , i need to truncate it to this format xxx-xx-6748 based on condition 

so it would be iif(Fields!TruncateSSN.Value=1, Fields!SSN.value, 'XXX-XX-left(4,Fields!SSN.value ) 

Can you help me correct this expression plz . 

Thanks