but nights is the function parameter of hotel_cost:

```
def hotel_cost(nights):
return 140 * nights
print hotel_cost(5) # will print 700
print nights # will give an error.
```

nights only exist inside the hotel_cost function, so using it outside the function will cause an error.

so in your trip_cost function:

```
def trip_cost(city, days):
return hotel_cost(....)
```

you have choice of 2 variables, days or city (trip_cost parameters)