def f(s, flag):
    if s == "":
        return 0
    if len(s) == 1:
        return int(s)
    a, b = int(s[0]), int(s[-1])
    v = a * (pow(10, (len(s) - 1)//2))
    if a <= b:
        v += f(s[1:-1], 0)
    if flag:
        v += f(str(10**(len(s) - 1) - 1), 0)
    return v

N = int(input())
ten = 10 ** 9
if N < ten + 1:
    print(0)
    exit()

a, b = N // ten, N % ten
if a > b:
    a -= 1
print(f(str(a), 1))