#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <complex>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <bitset>
#include <ctime>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <numeric>
#include <tuple>
#include <sstream>
#include <fstream>
#include <chrono>

using namespace std;
#define REP(i, n) for (int (i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int (i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for(int (i) = (a) - 1; (i) >= 0; (i)--)
#define FORR(i, a, b) for(int (i) = (a) - 1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template<typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define FIND(s, e) ((s).find(e) != (s).end())
#define EB emplace_back
template<typename T>inline bool chmax(T &a, T b){return (a < b ? a = b, true : false);}
template<typename T>inline bool chmin(T &a, T b){return (a > b ? a = b, true : false);}

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;
const long double EPS = 1e-9;

LL n;
const LL num = 1e9 + 1;

int main() {
    cin >> n;
    if (n < num) {
        cout << 0 << endl;
        return 0;
    }
    LL high = 1e9 + 10, low = 0;
    while (high - low > 1) {
        const LL mid = (high + low) / 2;
        if (num * mid <= n) low = mid;
        else high = mid;
    }
    LL cnt = low;
    LL ans = 0;
    if (cnt < 10) {
        cout << cnt << endl;
        return 0;
    }
    //DEBUG(low)
    ++low;
    REP(i, to_string(low).size()) {
        if (i + 1 != to_string(low).size()) ans += 9 * pow(10, i / 2);
        else {
            LL nn = powl(10, ((int)to_string(low).size() - 1) / 2);
            LL ma = nn * 10;
            while (true) {
                string str = to_string(nn);
                string strbuf = str;
                REVERSE(strbuf);
                strbuf = str + strbuf;
                if (low >= stoll(strbuf)) ++ans;
                ++nn;
                if (nn == ma) break;
            }
            nn = ma / 10;
            while (true) {
                string str = to_string(nn);
                string strbuf = str;
                REVERSE(strbuf);
                REP(i, 10) {
                    string strbuf2 = str + (char)(i + '0') + strbuf;
                    if (low >= stoll(strbuf2)) ++ans;
                }
                ++nn;
                if (nn == ma) break;
            }
            break;
        }
    }
    cout << ans << endl;
}