Need help (сбалансированные скобки)

Question

def balanced(expression):
a=0
b=0
for i in expression:
if i == '(':
a+=1
b+=2
elif i == ')':
a-=1
b+=2
if a == 0 and b!=0:
return True
else:
return False

print(balanced(input()))

7 test can't get agree

Answer 1

Reverso Context balance (data structure)

Answer 2

What is this code's objective?