Hi Nur Fadhilah Binte Mohamad Noor,
Create a separate variable to calculate the C value.
At the beginning of the microflow, retrieve the list where you are taking input for C. Iterate the list and calculate the C value. Since you have a value of C,now you can directly pass it to your original variable.
I hope this helps !!!
Update :
Remove the filter operation inside the list. You Loop will only contain the Chane Variable.
Please find the screen shot for your Reference :
Your Final Varible will look like this :
$InputValue/ValueA+ $InputValue/ValueA + $CValue