input: Write a Python function to perform sentiment analysis using the NLTK Vader library. 
  assert sentiment_analysis("I love this product!") > 0
  assert sentiment_analysis("I hate this service.") < 0
  assert sentiment_analysis("This is an okay experience.") == 0
output: def sentiment_analysis(text):
      from nltk.sentiment.vader import SentimentIntensityAnalyzer
      import nltk
      nltk.download('vader_lexicon')
      
      sia = SentimentIntensityAnalyzer()
      score = sia.polarity_scores(text)['compound']
      
      if score > 0:
          return 1
      elif score < 0:
          return -1
      else:
          return 0