#include<bits/stdc++.h>
#define ll          long long
#define dl          double
#define pb          push_back
#define eb          emplace_back
#define mp          make_pair
#define F           first
#define S           second
#define endl        "\n"
#define rep(i,a,b)  for(i=a;i<b;i++)
#define all(v)      v.begin(),v.end()
#define allr(v)     v.rbegin(),v.rend()
#define mod         1000000007
#define CIN(V,s,n)  for(int i=s;i<n;i++){cin >> V[i];}
#define COUT(V,s,n) for(int i=s;i<n;i++){cout <<  V[i] << " ";} cout << endl;
#define fast        ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define CLEAR(V);   for(ll i=0;i<V.size();i++) V[i] = 0;
#define N           100005
using namespace std;

bool SbS(const pair<int,int> &a,const pair<int,int> &b)
{return (a.second < b.second);}

ll SUMD(ll n);
ll BS(vector<ll> &PS,ll s,ll e,ll ser);
ll MI(ll a, ll m);
bool P[2002LL];
void Sieve(int n=N);
int nCrModp(int n, int r, int p);
vector<ll> PF(ll n);
ll Powb(ll b,ll n);

bool isO(ll n)
{
    ll cd=0;
    while(n>0)
    {
        n = n/10;

        cd++;
    }
    if(cd%2)
        return true;
    else
        return false;
}

int main()
{
fast;
ll q=1;
//cin >> q;
while(q--)
{
   ll n,m,k,i,j,l,r,a,t,b,c,u,v,w,p;
   cin >> n ;

    ll ans = 0;
    for(i=1;i<=n;i++)
    {
        if(isO(i))
            ans++;
    }
    cout << ans;
   //cout << max(0LL,k-n+m)<< endl;

}
    return 0;
}

//*****************************************************************************************************************************************

ll Powb(ll b,ll n)
{
    if(n==0) return 1LL;
    if(n==1) return b;
    ll temp = Powb(b,n/2);
    if(n%2==0){return (temp*temp)%mod;}
    else{return (b*((temp*temp)%mod))%mod;}
}

ll SUMD(ll n)
{   ll sum=0;
    while(n>0){sum += n%10;n = n/10;}
    return sum;

}
ll BS(vector<ll> &PS,ll s,ll e,ll ser)
{
    if(s>e)
        return s;
    ll mid = (s+e)/2;
    if(PS[mid]==ser)
    {
        return mid;
    }
    else if(PS[mid] > ser)
    {
        return BS(PS,s,mid-1,ser);
    }
    else
        return BS(PS,mid+1,e,ser);
}
ll MI(ll a, ll m)
{
    ll m0 = m;
    ll y = 0, x = 1;
    if (m == 1)
      return 0;
    while (a > 1)
    {
        ll q = a / m;
        ll t = m;
        m = a % m, a = t;
        t = y;
        y = x - q * y;
        x = t;
    }
    if (x < 0)
       x += m0;
    return x;
}

void Sieve(int n)
{
    memset(P,true,sizeof(P));
    P[0] = false;
    P[1]= false;
    for(ll i=2;i*i<=n;i++)
    {
        if(P[i])
        {
            for(ll j=i*i;j<=n;j+=i)
            {
                P[j]=false;
            }
        }
    }

}

int nCrModp(int n, int r, int p)
{
    int C[r+1];
    memset(C, 0, sizeof(C));

    C[0] = 1;
    for (int i = 1; i <= n; i++)
    {
        for (int j = min(i, r); j > 0; j--)

            C[j] = (C[j] + C[j-1])%p;
    }
    return C[r];
}

vector<ll> PF(ll n)
{
    vector<ll> pf;
    if(n%2==0)
    {
        pf.pb(2LL);
        n = n/2;
        while(n%2==0)
            n = n/2;
    }
    for(ll i=3;i<=sqrt(n);i+=2)
    {
        ll flag=1;
        while(n%i==0)
        {
            if(flag)
            {
                pf.pb(i);
                flag=0;
            }
            n = n/i;
        }
    }
    if(n>2)
        pf.pb(n);

    //COUT(pf,0,pf.size());
    return pf;
}




Measured memory_bytes: 256