Calculator inside the expense field

When splitting an expense into multiple categories, I need to have the total amount calculated in advance. Although this amount is deducted from the total, I think it would be nice to have a basic calculator in the fields for splitting transactions as well (e.g., for a purchase at a shopping center, I’ll enter the amount 30 and then split the transaction further into, say, drinks and drugstore items. For drinks, I’d add up, say, Coca-Cola + Fanta (3 + 2.99); for drugstore items, say, napkins and laundry detergent (0.5 + 12). The resulting totals would be displayed automatically, and I wouldn’t have to calculate them in advance.