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The problem statement states that the account (y) should have enough money for both the transaction amount x and bank charges 0.5. i.e. x+ 0.5 >= previous balance(y) in bank. As you are checking only in the cases when x > y, you will give wrong (actually negative ) answer (-0.2) for the case x=10 and y=10.3. This satisfies x>y but x+0.5 is not greater than 10.3.