def pickup(S,ans):
    l = len(S)
    if l<2:
        return ans
    check_dif = False
    #print(S)
    for i in range(l-1):
        if S[i] != S[i+1]:
            S.pop(i)
            S.pop(i)
            #print(S)
            check_dif = True
            ans += pickup(S,ans)
            break
    if not check_dif:
        return ans
    else:
        return ans+2
    
S=input()
l = len(S)
S_list = [S[i] for i in range(l)]
ans =0
print(pickup(S_list,ans))