def addBinary(a, b):
            if len(a)==0: return b
            if len(b)==0: return a
            if a[-1] == '1' and b[-1] == '1':
                return addBinary(addBinary(a[0:-1],b[0:-1]),'1')+'0'
            if a[-1] == '0' and b[-1] == '0':
                return addBinary(a[0:-1],b[0:-1])+'0'
            else:
                return addBinary(a[0:-1],b[0:-1])+'1'

print(addBinary('1010','0011'))