#include <bits/stdc++.h>

using namespace std;
using ull = unsigned long long;

int INF = 1000000000;

int main(){
    ull N;
    cin >> N;
    ull ans = 0;
    ull now = 0;
    ull check = 10;
    while(N >= check){
        ans += now + 55;
        now += 100;
        check += 10;
    }
    for(ull j = check - 9;j <= N;j++)ans += j;
    cout << ans << endl;
}