error in ns2 program

iam doing project in ns2 ………..
so i have did one program …..but some error occurring …..so can you please anyone help me to correct this error ………..
my program of udp is
#Create a simulator object
set ns [new Simulator]

$ns color 1 Blue
$ns color 1 Red
$ns color 2 White
#Open the nam trace file
set nf [open out.nam w]
$ns namtrace-all $nf

#Define a ‘finish’ procedure
proc finish {} {
global ns nf
$ns flush-trace
#Close the trace file
close $nf
#Execute nam on the trace file
exec nam out.nam &
exit 0
}
#Create four nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]

#Create links between the nodes
$ns duplex-link $n0 $n1 0.2Mb 10ms DropTail
$ns duplex-link $n0 $n2 0.2Mb 10ms DropTail
$ns duplex-link $n1 $n3 0.2Mb 10ms DropTail
$ns duplex-link $n2 $n3 0.2Mb 10ms DropTail
set udp [new Agent/UDP]
$udp set class_ 1
set sink [new Agent / Null]
$ns attach-agent $n0 $udp
$ns attach-agent $n3 $sink
$ns connect $udp $sink
set cbr [new Application / Trafic / CBR]
$cbr set packetSize _ 1000
$cbr attach-agent $udp
$ns at 1.0 “$cbr start”
$ns at 29.0 “$cbr stop”
$ns at 30.0 “finish”
proc finish {} {
global _ns tracefd namtrace
close $tracefd
close $namtrace
}
puts “starting simulation…”

$ns run