Multi - Select Lookup Field

Multi - Select Lookup Field

Hello,
I am trying to add values of Multi-Select Lookup field and store them in another field
Value should be updated On User Input
Following is the code i used to add values
  2. var = 0.0;
  3. for each  rec in Quantity
  4. {
  5. if(Quantity.size()>0)
  6. {
  7. var = var + Quantity.todecimal();
  8. }
  9. }
  10. input.Total = var;

Values inside the Multi-Select lookup field are all decimal values

Following is the error i received :