Hi
In crystal Shared variables will store the values and also fetch the values, these variables are global variables and can be used in your main report formulas.
Sample formula to store value (in your case sub report value )
Whileprintingrecords; // This formula store value to a 'xx' shared variable.
Shared Numbervar xx;
xx:={@AV Calculation for Both S1_S2};
In your main report to fetch the value, use the below formula :
Whileprintingrecords;
Shared Numbervar xx;
This will display your sub report value in main repor. Make sure that your sub report should get processed before you display the variable value.