def main():
    S = input()
    cnt = 0
   
    if len(set(S)) == 1 :
        print(0)
    else:
        l_past  = None
        l_new = None
        block = list(S)

        for i in range(10**5):
            block = [ a for a in block if a != "-"]
            l_new = len(block)

            if l_past == l_new :
                break

            for i in range(len(block) - 1):
                if (block[i] == "0" and block[i+1] == "1") or (block[i] == "1" and block[i+1] == "0"):
                    cnt += 2
                    block[i] = "-"
                    block[i+1] = "-"

            l_past = l_new

        print(cnt)

if __name__ == "__main__":
    main()