input: Write a python function to authenticate and login an existing user.
  assert login('john_doe', 'securepassword123')==True
  assert login('jane_doe', 'wrongpassword')==False
  assert login('user_not_exist', 'any_password')==False
output: def login(username, password):
      # Example user data store; in real applications, use a database
      user_data = {
          'john_doe': 'securepassword123',
          'jane_doe': 'password456'
      }
      
      # Check if the username exists and the password matches
      if username in user_data and user_data[username] == password:
          return True
      return False