The water balance sheet

Two dictionaries give the water that a person gains and loses in a day, in litres. You will add them up, work out the urine that keeps the balance, and say whether the water is short.

The idea. The body water stays the same when the gains equal the losses. The kidneys cannot pass less than about 0.5 L of urine a day.

Your tasks.

1. Finish total. It adds the values of a dictionary.
2. Finish urine_needed. It gives the litres of urine that make the losses equal to the gains.
3. Finish short_of_water. It gives True when the urine that is needed is less than 0.5 L.