only sumif checkbox is checked?

How do I only sum the amounts for Susan that are checked?

=if(AND(Sheet1!A:A="Susan",Sheet2!N:N=true,sum(Sheet2!M:M,""))))

or

=sumif(AND(Sheet1!A:A="Susan",Sheet2!N:N=true),Sheet2!M:M,""))))

I believe the above snippets are saying that all N:N checkboxes must be checked to be true, or will return false. Anyway they're not working...

Sheet1      Sheet2             
Column A    Column M     Column N
 Tom         100         (un-checked)
 Susan       150         (checked)
 Susan       75          (un-checked)
 Tom         25          (checked)
 Susan       50          (checked)