//#include "stdafx.h"
#include <iostream>
#include <vector>

using namespace std;

#define int long long

int pow(int x, int y) {
	int p = 1;
	for (int i = 0; i < y; i++)
	{
		p *= x;
	}
	return p;
}

#define KEY 1000000001
int digit(int n){
	for (int i = 1; i < 18; i++)
	{
		if (pow(10,i) > n) {
			return i;
		}
	}
	return 18;
}

int rightCount(int digit) {
	int n = digit - 10;
	int sum = 0;
	for (int i = 1; i <= n; i++)
	{
		sum += pow(9, i % 2);
	}
	return sum;
}

void genkeys(int keys[], int pd) {
	//cout << pd << endl;
	if (pd == 1) { keys[0] = 1; return; }
	else if (pd == 2) { keys[0] = 11; keys[1] = 1; return; }
	for (int i = 0; i < pd / 2; i++)
	{
		keys[i] = pow(10, pd - 1 - i) + pow(10, i);
	}
	if (pd % 2 == 1) keys[pd / 2] = pow(10, pd / 2);
}

int genNum(int keys[], int keyCount[], int pd) {
	int num = 0;
	for (int i = 0; i < pd / 2; i++)
	{
		num += keys[i] * keyCount[i];
	}
	if (pd % 2 == 1) num += keys[pd / 2] * keyCount[pd / 2];
	return num * KEY;
}

int check(int n) {
	int d = digit(n);
	//cout << d << endl;
	if (d < 10) return 0;
	int right = rightCount(d);
	//cout << right << endl;
	int pd = d - 9;
	int keys[5];
	genkeys(keys, pd);
	int keyCount[5] = { 1, 0, 0 ,0 ,0 };
	int ex = 0;
	int keysCount = (pd / 2) + (pd % 2 == 0 ? 0 : 1);

	while (true) {
		if (n <= genNum(keys, keyCount, pd)) {
			break;
		}
		//cout << genNum(keys, keyCount, pd) << endl;
		ex++;
		keyCount[keysCount - 1]++;
		for (int i = keysCount - 1; 0 < i; i--) {
			if (keyCount[i] == 10)
			{
				keyCount[i] = 0;
				keyCount[i - 1]++;
			}
			else {
				break;
			}
		}
	}
	return (ex + right);
}

signed main() {
	int t;
	cin >> t;
	cout << check(t) << endl;
	return 0;
}