#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)




#define rrep(i,a,b) for(int i=a;i>=b;i--)
typedef long long ll;
ll N;
//-----------------------------------------------------------------
int main() {
	cin >> N;
	ll mo = 1000000001;

	int dig = 0;
	ll t = N;
	while (0 < t) dig++, t /= 10;
	
	ll ans = 0;
	rrep(i, 8, 1) {
		ll c = 0;
		int d = i + 2 * (9 - i);
		if (d < dig) {
			// 全部OK
			int j = 9 - i;

			if (j == 1) {
				c = 9;
			}
			else if(j % 2 == 0){
				int k = j / 2 - 1;
				c = 9;
				rep(kk, 0, k) c *= 10;
				ans += c;
			}
			else {
				int k = j / 2;
				c = 9;
				rep(kk, 0, k) c *= 10;
				ans += c;
			}
		} else if (d == dig) {
			// 一部OK
			int j = 9 - i;
			
			string n = to_string(N);
			int ma = 0;
			rep(i, 0, j) ma = ma * 10 + n[i] - '0';

			int mi = 0;
			rep(i, 0, j) mi = mi * 10 + 1;

			rep(x, mi, ma) {
				string s = to_string(x);
				bool ok = true;
				rep(i, 0, s.length() / 2 + 1) {
					int j = s.length() - 1 - i;
					if (s[i] != s[j]) ok = false;
				}
				if (ok) c++;
			}

			ll th = ma;
			rep(i, 0, d - j * 2) th *= 10;
			int y = ma;
			while (0 < y) {
				th = th * 10 + y % 10;
				y /= 10;
			}

			if (th <= N) c++;
			
			//cout << th << endl;

			/*cout << j << endl;
			cout << ma << endl;
			cout << mi << endl;*/
		}

		ans += c;
		//printf("%d -> %lld\n", d, c);
	}
	cout << ans << endl;
}