import collections
s=list(map(str,input()))
c=0
cl=collections.Counter(s)
while cl["1"]!=0 and cl["0"]!=0 :
    for i in range(1,len(s)):
        if s[i-1] != s[i]:
            del s[i-1]
            del s[i-1]
            c+=2
            cl=collections.Counter(s)
            break
        else:
            pass

print(c)
    
