How to Resolve Errors

from traillets.config.application import logging
def AND(a,b):
  A = a * b
  return A
 
def OR(a,b):
   if a == 1:
     if b == 1:
        A = 1
     else:
        A = 0

def NOT(a):
  if a == 1:
    A = 0
  if a == 0:
    A = 1
  return A

def ha(a,b):
    A1 = OR(a,b)
    keta = AND(a,b)
    A2 = NOT (keta)
    A = AND (A1, A2)
    return A, keta

deffa(a,b,x):
  A1,keta1 = ha(a,b)
  A,keta2 = ha(A1,x)
  keta=OR (keta1, keta2)
  return A, keta

a = input ("binary number")
b = input ("the binary plus")
k=len(a)
k = k - 1
x = 0
goukei=""
while k>=0:
  ha = int(a(k))
  fa = int(b(k))
  A,keta=fa(a,b,x)
  goukei=str(A)+goukei
  x = keta
  k = k - 1
  goukei=str(x)+goukei
  print(a, "+", b, "=", gookei)

TypeError Traceback (most recent call last)
<ipython-input-30-1544c3673a72>in<module>
     39goukei=""
     40 while k>=0:
--- >41 ha = int(a(k))
     42 fa = int(b(k))
     43 A, keta = fa(a, b, x)
TypeError: 'str' object is not callable