#include <bits/stdc++.h>
using namespace std;
#define FOR(i, n) for(int i = 0; i < (n); i++)
#define MEM(a, x) memset(a, x, sizeof(a))
#define ALL(a) a.begin(), a.end()
#define UNIQUE(a) a.erase(unique(ALL(a)), a.end())
typedef long long ll;
typedef pair<int, int> P;

ll x;

int num(int sz) {
	return 9 * pow(10, (sz-10)/2);
}

int main(int argc, char const *argv[]) {
	ios_base::sync_with_stdio(false);
	cin >> x;
	if (x < 1000000001) {
		cout << 0 << endl;
		return 0;
	}
	string s = to_string(x);
	int n = s.size();
	if (n == 19) {
		n--;
		x--;
	}
	int ret = 0;
	for (int i = 10; i < n; i++) ret += num(i);
	for (int i = pow(10, (n-10)/2); i < pow(10, (n-10)/2+1); i++) {
		string t1 = to_string(i), t2 = to_string(i);
		reverse(ALL(t2));
		if (n & 1) t1 += t2;
		else t1 += t2.substr(1);
		int nn = t1.size();
		string t = t1 + string(n-2*nn, '0') + t1;

		if (stoll(t) <= x) ret++;
	}
	cout << ret << endl;
	return 0;
}