[racket] begin vs +

Hello,

This code prints "hi" twice,

(define d (box #f))

(+ (begin (let/cc k (begin (set-box! d k) 3))) (begin (print "hi") 9)) 

((unbox d) 0)

whereas this prints "hi" once

(define d (box #f))

(begin (begin (let/cc k (begin (set-box! d k) 3))) (print "hi")) 

((unbox d) 0)

Why?

Keiko