#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include <bits/stdc++.h>
using namespace std;
using i32 = int; using i64 = long long int; using f64 = double; using str = string;
template <typename T> using vec = vector<T>;
template <typename T> using heap = priority_queue<T, vec<T>, greater<T>>;
#define times(n, i) for (i32 i = 0; i < (n); ++i)
#define range(a, b, i) for (i32 i = (a); i < (b); ++i)
#define upto(a, b, i) for (i32 i = (a); i <= (b); ++i)
#define downto(a, b, i) for (i32 i = (a); i >= (b); --i)
#define all(xs) (xs).begin(), (xs).end()
#define sortall(xs) sort(all(xs))
#define reverseall(xs) reverse(all(xs))
#define uniqueall(xs) (xs).erase(unique(all(xs)), (xs).end())
#define even(x) (((x) & 1) == 0)
#define odd(x) (((x) & 1) == 1)
#define append emplace_back
const i64 MOD = 1000000007;

i64 F = 1000000001;
i64 n;

i64 count(i64 maxv) {
    i32 len = 1;
    i32 acc = 1;
    while (acc*10 <= maxv) {
        acc *= 10;
        len++;
    }
    i64 ret = 0;
    if (len > 1) {
        ret += count(pow(10, len-1)-1);
    }

    i64 t, v;
    upto(1, i32(pow(10, (len+1)/2))-1, h) {
        t = h;
        if (t - (t/10*10) == 0) continue;
        v = t;
        downto(len/2, 1, e) {
            v += (t - (t/10*10)) * pow(10, e+(len+1)/2-1);
            t /= 10;
        }
        if (v <= maxv) {
            ret++;
        }
    }
    return ret;
}

i32 main()
{
    str s;
    cin >> s;
    n = stoll(s);
    cout << count(n/F) << endl;
    return 0;
}