input: Write a function to manage a checking account with deposit and withdraw methods.
  assert ca.deposit(100) == 200
  assert ca.withdraw(50) == 150
  assert ca.withdraw(200) == "Insufficient funds"
output: class CheckingAccount:
      def __init__(self, initial_balance=0):
          self.balance = initial_balance
      
      def deposit(self, amount):
          self.balance += amount
          return self.balance
  
      def withdraw(self, amount):
          if amount > self.balance:
              return "Insufficient funds"
          self.balance -= amount
          return self.balance
  
  # Example usage:
  ca = CheckingAccount(100)