input: Write a python function to check if the login credentials are correct.
  assert check_login('user1', 'password') == True
  assert check_login('user2', 'password') == True
  assert check_login('user1', 'wrongpassword') == False
  assert check_login('nonexistentuser', 'password') == False
output: def check_login(username, password):
      credentials = {
          'user1': 'password',
          'user2': 'password'
      }
      
      if username in credentials and credentials[username] == password:
          return True
      return False