import sys
input = sys.stdin.readline
s = input()
count = 0

if s.count("0") != 0 and s.count("1") != 0:
    if s.count("0") > s.count("1"):
        print(2*s.count("1"))
    elif s.count("1") > s.count("0"):
        print(2*s.count("0"))
    else:
        print(len(s))
else:
    print(0)
