s = input()
if len(s) == 1:
    print(0)
else:
    count0 = 0
    count1 = 0
    for i in range(len(s)):
        if s[i:i+1] == "0":
            count0 += 1
        else:
            count1 += 1
    print(2*max([count0,count1]))