expressions in SSRS

Experts,

I have two datasets in SSRS. There is a column in each of the two datasets where I want to get the sum of this column in each sets to be obtained and again the two sums be summed up again.

So, like SUM(sum(column), Dataset1)   +  sum(column), Dataset2)) ...... I want to keep the resulting value in a cell.

Cheers

ebro