def unification(n,s):
    rc = 0
    bc = 0

    for c in s:
        if c == '0':
            rc += 1
        elif c == '1':
            bc += 1
        else:
            break

    return min(rc,bc) * 2

if __name__ == '__main__':
    s = raw_input()
    n = len(s)
    _s = [i for i in s]

    print unification(n,_s)