input: Write a python function to find the oldest movie in a list of movies.
  assert oldest_movie([{"title": "Movie A", "releaseYear": 1995}, {"title": "Movie B", "releaseYear": 1980}, {"title": "Movie C", "releaseYear": 2000}]) == 1980
  assert oldest_movie([{"title": "Movie X", "releaseYear": 2010}, {"title": "Movie Y", "releaseYear": 2005}]) == 2005
  assert oldest_movie([{"title": "Movie D", "releaseYear": 2020}, {"title": "Movie E", "releaseYear": 2020}, {"title": "Movie F", "releaseYear": 2018}]) == 2018
output: def oldest_movie(movies):
      return min(movie["releaseYear"] for movie in movies)