function GetNum(len:integer):string;
var i:integer;
s:string;
function GetRanChar:string;
var c:char;
begin
repeat
c:=char(random(75)+47);
until c in ['0'..'9','A'..'Z'];
Result:=string(c);
end;
begin //构造SN
randomize;
s:='';
for i:=1 to len do
begin
s:=s+GetRanChar;
end;
Result:=s;
end;