s = list(str(input()))
n = len(s)
ans = 0
s2 = s
for i in range(n):
    for j in range(n):
        if s2[i:i+2] == ['0','1'] or  s2[i:i+2] == ['1','0']:
            ans += 2
            s2.remove(s2[i])
            s2.remove(s2[i+1])
            continue
        else:
            continue