#include <bits/stdc++.h>
using namespace std;
long long n, mx, now, ans;
int main() {
  scanf("%lld", &n);
  mx = 2 * n - 1;
  while (now * 10 + 9 <= mx) now = now * 10 + 9;
  for (long long k = now; k <= mx; k += now + 1) {
    long long d = max(k - n, 1ll), u = max(k - d, d);
    ans += (u - d + 1) / 2;
  }
  printf("%lld\n", ans);
  return 0;
}
