s = input()
ans = 0
while '01' in s or '10' in s:
    # print('yes')
    if '01' in s:
        s = s.replace('01', '', 1)
        ans += 2
    if '10' in s:
        s = s.replace('10', '', 1)
        ans += 2
    # print(s, ans)

print(ans)