def build_url(city: str, unit: str, key: str) -> str:
	"""
	Build API call url.

	Parameters:
	city (str): City name.
	unit (str): Units system (metric or imperial).
	key (str): API key.

	Returns:
	full_url (str): API call URL.
	"""
	api_url = 'http://api.openweathermap.org/data/2.5/weather?q='
	if unit == 'm':
		units = 'metric'
	else:
		units = 'imperial'
	full_url = api_url + city + '&mode=json&units=' + units + '&APPID=' + key
	return full_url