#define DEBUG 0
#include <algorithm>
#include <cmath>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <set>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using pll = pair<ll, ll>;
using tll = tuple<ll, ll, ll>;
#define pb push_back
#define all(v) v.begin(), v.end()
#define for1(i, n) for (ll i = 0; i < (n); i++)
#define for2(i, m, n) for (ll i = (m); i < (n); i++)
#define for3(i, m, n, d) for (ll i = (m); i < (n); i += (d))
#define INF (1LL << 60)
#if DEBUG
#define dump(v1) dump_1(#v1, v1)
#define dump2(v1, v2) dump_2(#v1, v1, #v2, v2)
#define dump3(v1, v2, v3) dump_3(#v1, v1, #v2, v2, #v3, v3)
#else
#define dump(v1)
#define dump2(v1, v2)
#define dump3(v1, v2, v3)
#endif
#if DEBUG
template <typename T> void dump_1(const string &n1, const T &v1) {
    cerr << n1 << ": " << v1 << endl;
}
template <typename T1, typename T2>
void dump_2(const string &n1, const T1 &v1, const string &n2, const T2 &v2) {
    cerr << n1 << ": " << v1 << "  " << n2 << ": " << v2 << endl;
}
template <typename T1, typename T2, typename T3>
void dump_3(const string &n1, const T1 &v1, const string &n2, const T2 &v2,
            const string &n3, const T3 &v3) {
    cerr << n1 << ": " << v1 << "  " << n2 << ": " << v2 << "  " << n3 << ": "
         << v3 << endl;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &v) {
    os << "(" << v.first << ", " << v.second << ")";
    return os;
}
template <typename T1, typename T2, typename T3>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3> &v) {
    os << "(" << get<0>(v) << ", " << get<1>(v) << ", " << get<2>(v) << ")";
    return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "[";
    for (auto it = v.begin(); it != v.end(); it++) {
        if (it != v.begin()) {
            os << ", ";
        }
        os << *it;
    }
    os << "]";
    return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &v) {
    os << "{";
    for (auto it = v.begin(); it != v.end(); it++) {
        if (it != v.begin()) {
            os << ", ";
        }
        os << *it;
    }
    os << "}";
    return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &v) {
    os << "{";
    for (auto it = v.begin(); it != v.end(); it++) {
        if (it != v.begin()) {
            os << ", ";
        }
        os << it->first << ":" << it->second;
    }
    os << "}";
    return os;
}
#endif // DEBUG
void Yes(void) { cout << "Yes" << endl; }
void No(void) { cout << "No" << endl; }
void YES(void) { cout << "YES" << endl; }
void NO(void) { cout << "NO" << endl; }
template <typename T> void print(const T &v) { cout << v << endl; }
//---------------------------------------------------------
ll calc(ll n) {
    ll len = to_string(n).length();
    return len % 2 == 1;
}
void solve() {
    ll N;
    cin >> N;
    ll ans = 0;
    for2(i, 1, N + 1) {
        if (calc(i)) {
            ++ans;
        }
    }
    print(ans);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    solve();
}


Measured memory_bytes: 256