I am receiving this error (typeerror: unsupported operand type(s) for +=: 'builtin_function_or_method' and 'int') when trying to run this code

def calculate():
    number_1 = int(input('Enter your first number: '))
    number_2 = int(input('Enter your second number: '))
    
    operation = input('''
    Enter an operator:
    + for addition
    - for subtraction
    * for multiplication
    / for division
    ''')
    
    if operation == '+':
      sum = number_1 + number_2
      print('{} + {} = '.format(number_1, number_2))
      print(sum)
    
    elif operation == '-':
      diff = number_1 - number_2
      print('{} - {} = '.format(number_1, number_2))
      print(diff)
    
    elif operation == '*':
      product = number_1 * number_2
      print('{} * {} = '.format(number_1, number_2))
      print(product)
    
    elif operation == '/':
      quotient = number_1 / number_2
      print('{} / {} = '.format(number_1, number_2))
      print(quotient)
    
    else:
        print('You have not typed a valid operator, please run the program again.')
      
calculate()

def recal(): 
    c = int(input('Enter the number: '))

    operation2 = input('''
    Enter an operator:
    + for addition
    - for subtraction
    * for multiplication
    / for division
    ''')

    if operation2 == '+':
      answer = sum + c 
      print('{} + {} = '.format(sum, c))
      print(answer)

    elif operation2 == '-':
      answer2 = diff - c 
      print('{} - {} = '.format(diff, c))
      print(answer2)

    elif operation2 == '*':
      answer3 = product * c
      print('{} * {} = '.format(product, c))
      print(answer3)

    elif operation2 == '/':
      answer4 = quotient / c
      print('{} / {} = '.format(quotient, c))
      print(answer4)

    else:
      print('You have not typed a valid operator, please run the program again.')

while True:    
    calc_again = input('''
Do you want to compute again?
Please type Y for YES or N for NO.
Type C if you want to continue.''')
    if calc_again == 'Y':
        calculate()

    elif calc_again == 'N':
        print('Thank You')
        

    elif calc_again == 'C':
      recal()
    
    else:
      print('END')