Let us follow Alice's wallet.

She starts with $a$ hryvnias. Bob and Svitlana pay back their debts, so $b$ and $c$ hryvnias are added. Then Alice pays her debt to Dmytro, and $d$ hryvnias are subtracted.

In total we get

The statement guarantees that the result is non-negative, so the case of «not enough money» needs no separate handling. All values are at most $100$, so there is no overflow and no real numbers here.