i = 0
total = 0
while i < len(S):
    if S[i:i+2] == '01'or S[i:i+2] == '10':
        Sa = S[:i]
        Sb = S[i+2:]
        S = Sa+Sb
        total += 1
        i -= 1
    else:
        i += 1
print(total*2)