How to use parameters

iam developing the crystal report there was small problem iam facing
please help me
How to use parameter of main report into the subreport
example
@.startdatetime is a parameter in main report
ia want to use the same parmeter in subreportcreate startdatetime parameter in subreport as well, then like it with main reports parameter.

i think i already suggested this to you.|||Thanks for reply

But we are getting six parameter while running the report
six parameters are
3 from sub reports(@.agencyid,@.startdatetime,@.Enddatetime)
3 from main reports(@.agencyid,@.startdatetime,@.Enddatetime)

while running the report it asking for
@.agencyid for subreport name
@.startdatetime for subreport name
@.Enddatetime for subreport name
@.agencyid
@.startdatetime
@.Enddatetime

but i requried only three parameter to be displayed to enter the values
please suggest me to solve the problem|||yes after linking it will ask only for main report parameters.|||You can make use of shared variables. Read it in help file