question about rule detect nmap scan

["Vuong D. Chieu" <vdchieu () vncert vn>]

you can test help me rule detect scan nmap ?
this is my rule but it is not working

alert tcp any any -> any any (sid:1000005; gid:1; flow:stateless; ack:0; flags:S; ttl:>220; priority:1; msg:"nmap 
scan"; classtype:network-scan; rev:1; )
----------------------------------------
Vuong Dinh Chieu (Mr.)
Vietnam Computer Emergency Response Team (VNCERT)
Ministry of Information and Communications (MIC)
Add: 18 Nguyen Du, Hanoi       Website: http://www.vncert.gov.vn
Tel: +84-4-3640-4424                Mobile: +84-97 993 1293


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