#include <bits/stdc++.h>

#define REP(i, x, n) for (int i = x; i < (int)(n); i++)
#define rep(i, n) REP (i, 0, n)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) (int)(x.size())
#define popcount(x) __builtin_popcount(x)
#define popcountll(x) __builtin_popcountll(x)
#define uniq(x) x.erase(unique(x.begin(), x.end()), x.end())
#define F first
#define S second
#define mp make_pair
#define eb emplace_back

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

const int INF = 1 << 29;

vector<ll> plist;

ll toll(string s) {
    ll res = 0;
    rep (i, s.size()) {
        res = res * 10 + (int)(s[i] - '0');
    }
    return res;
}

void dfs(string p) {
    if (p.size() > 9) return;

    if (p.size() != 0 && p[0] != '0') {
        plist.push_back(toll(p));
    }

    rep (i, 10) {
        string c = string(1, (char)('0' + i));
        dfs(c + p + c);
    }
}

int main() {
    ull n;
    cin >> n;

    // even
    dfs("");

    // odd
    rep (i, 10) {
        dfs(string(1, (char)('0' + i)));
    }

    sort(all(plist));

    int ans = 0;
    rep (i, plist.size()) {
        ull v = (ull) 1e9 * plist[i] + plist[i];
        if (v <= n) ans++;
        else break;
    }

    cout << ans << endl;
}