dsum in form

Hi all,

reg   amt   amt total
1       10        10*50=500
1      100        100*50=5000
1       200        200*50=10000
2       2             2*50=100
2        3            3*50=150


pls see the above i was design the subform like this, but i was facing
the problem like.

in subform footer i was insert the textbox and sum the amt total is
not working.

note:amt is only display in form not store in table.

i need the result=sum([amt total])

can any one guide me for this issue.

Regards,

deen