<gh_stars>0
# S = input()
# ans = 0
# while True:
#     if S.count("01") == 0 and S.count("10") == 0:
#         break

#     if S.count("01") > 0:
#         ans += S.count("01")*2
#         S = S.replace("01", "")

#     if S.count("10") > 0:
#         ans += S.count("10")*2
#         S = S.replace("10", "")

# print(ans)

S = input()
print(min(S.count("0"),S.count("1"))*2)

# S = input()
# ans = 0
# while True:
#     if S.count("01") == 0 and S.count("10") == 0:
#         break

#     if S.count("01") > 0:
#         ans += S.count("01")*2
#         S = S.replace("01", "")

#     if S.count("10") > 0:
#         ans += S.count("10")*2
#         S = S.replace("10", "")

# print(ans)

S = input()
print(min(S.count("0"),S.count("1"))*2)
# S = input() # ans = 0 # while True: #     if S.count("01") == 0 and S.count("10") == 0: #         break #     if S.count("01") > 0: #         ans += S.count("01")*2 #         S = S.replace("01", "") #     if S.count("10") > 0: #         ans += S.count("10")*2 #         S = S.replace("10", "") # print(ans)