#include <bits/stdc++.h>
#include <sys/time.h>
using namespace std;

// hamko utils
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define repi(i, a, b) for (long long i = (long long)(a); i < (long long)(b); i++)
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second
#define mt make_tuple
#define mp make_pair
template <class T1, class T2>
bool chmin(T1 &a, T2 b)
{
  return b < a && (a = b, true);
}
template <class T1, class T2>
bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<ll, ll>;
using ld = long double;
using vld = vector<ld>;
using vi = vector<int>;
using vvi = vector<vi>;
vll conv(vi &v)
{
  vll r(v.size());
  rep(i, v.size()) r[i] = v[i];
  return r;
}

inline void input(int &v)
{
  v = 0;
  char c = 0;
  int p = 1;
  while (c < '0' || c > '9')
  {
    if (c == '-')
      p = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
  {
    v = (v << 3) + (v << 1) + c - '0';
    c = getchar();
  }
  v *= p;
} // これを使うならば、tieとかを消して！！
template <typename T, typename U>
ostream &operator<<(ostream &o, const pair<T, U> &v)
{
  o << "(" << v.first << ", " << v.second << ")";
  return o;
}
template <size_t...>
struct seq
{
};
template <size_t N, size_t... Is>
struct gen_seq : gen_seq<N - 1, N - 1, Is...>
{
};
template <size_t... Is>
struct gen_seq<0, Is...> : seq<Is...>
{
};
template <class Ch, class Tr, class Tuple, size_t... Is>
void print_tuple(basic_ostream<Ch, Tr> &os, Tuple const &t, seq<Is...>)
{
  using s = int[];
  (void)s{0, (void(os << (Is == 0 ? "" : ", ") << get<Is>(t)), 0)...};
}
template <class Ch, class Tr, class... Args>
auto operator<<(basic_ostream<Ch, Tr> &os, tuple<Args...> const &t) -> basic_ostream<Ch, Tr> &
{
  os << "(";
  print_tuple(os, t, gen_seq<sizeof...(Args)>());
  return os << ")";
}
ostream &operator<<(ostream &o, const vvll &v)
{
  rep(i, v.size())
  {
    rep(j, v[i].size()) o << v[i][j] << " ";
    o << endl;
  }
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const vector<T> &v)
{
  o << '[';
  rep(i, v.size()) o << v[i] << (i != v.size() - 1 ? ", " : "");
  o << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const deque<T> &v)
{
  o << '[';
  rep(i, v.size()) o << v[i] << (i != v.size() - 1 ? ", " : "");
  o << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const set<T> &m)
{
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const unordered_set<T> &m)
{
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T, typename U>
ostream &operator<<(ostream &o, const map<T, U> &m)
{
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it << (next(it) != m.end() ? ", " : "");
  o << "]";
  return o;
}
template <typename T, typename U, typename V>
ostream &operator<<(ostream &o, const unordered_map<T, U, V> &m)
{
  o << '[';
  for (auto it = m.begin(); it != m.end(); it++)
    o << *it;
  o << "]";
  return o;
}
vector<int> range(const int x, const int y)
{
  vector<int> v(y - x + 1);
  iota(v.begin(), v.end(), x);
  return v;
}
template <typename T>
istream &operator>>(istream &i, vector<T> &o)
{
  rep(j, o.size()) i >> o[j];
  return i;
}
template <typename T, typename S, typename U>
ostream &operator<<(ostream &o, const priority_queue<T, S, U> &v)
{
  auto tmp = v;
  while (tmp.size())
  {
    auto x = tmp.top();
    tmp.pop();
    o << x << " ";
  }
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const queue<T> &v)
{
  auto tmp = v;
  while (tmp.size())
  {
    auto x = tmp.front();
    tmp.pop();
    o << x << " ";
  }
  return o;
}
template <typename T>
ostream &operator<<(ostream &o, const stack<T> &v)
{
  auto tmp = v;
  while (tmp.size())
  {
    auto x = tmp.top();
    tmp.pop();
    o << x << " ";
  }
  return o;
}
template <typename T>
unordered_map<T, ll> counter(vector<T> vec)
{
  unordered_map<T, ll> ret;
  for (auto &&x : vec)
    ret[x]++;
  return ret;
};
string substr(string s, P x) { return s.substr(x.fi, x.se - x.fi); }
void vizGraph(vvll &g, int mode = 0, string filename = "out.png")
{
  ofstream ofs("./out.dot");
  ofs << "digraph graph_name {" << endl;
  set<P> memo;
  rep(i, g.size()) rep(j, g[i].size())
  {
    if (mode && (memo.count(P(i, g[i][j])) || memo.count(P(g[i][j], i))))
      continue;
    memo.insert(P(i, g[i][j]));
    ofs << "    " << i << " -> " << g[i][j] << (mode ? " [arrowhead = none]" : "") << endl;
  }
  ofs << "}" << endl;
  ofs.close();
  system(((string) "dot -T png out.dot >" + filename).c_str());
}
size_t random_seed;
namespace std
{
using argument_type = P;
template <>
struct hash<argument_type>
{
  size_t operator()(argument_type const &x) const
  {
    size_t seed = random_seed;
    seed ^= hash<ll>{}(x.fi);
    seed ^= (hash<ll>{}(x.se) << 1);
    return seed;
  }
};
}; // namespace std
#define ldout fixed << setprecision(40)

#define EPS (double)1e-14
#define INF (ll)1e18
#define mo (ll)(1e9 + 7)
// end of hamko utils

int main()
{
  int N;
  cin >> N;
  int digit = 0;
  int p = N;
  int ans = 0;
  while (p > 0)
  {
    p /= 10;
    ++digit;
  }
  cerr << digit << endl;
  if (digit % 2 == 0){
    --digit;
    for(int i = 0; i < digit; i+=2){
      int q = i;
      ans += 9*pow(10,q);
      --q;      
    }
    
  } else {
    for(int i = 0; i < digit; i+=2){
      int q = i;
      ans += 9*pow(10,q);
      --q;      
    }
    ans += N - pow(10,digit)+1;
  }
  cout << ans << endl;
}



Measured memory_bytes: 256