#include <bits/stdc++.h>
using namespace std;
typedef long long lint;

lint p[10];

int main(){
    lint N; cin >> N;

    p[0] = 1;
    for(int i=1;i<10;i++) p[i] = p[i-1] * 10;

    N = N / lint(1e9+1);
    string S = to_string(N);

    lint ans = 0;
    int k = S.size();
    for(int i=1;i<k;i++) ans += 9 * p[(i-1)/2];

    for(int i=2;i<=(k+1)/2;i++) if(S[i-1]!='0' and S[k-i]!='0') ans += (S[i-1]-'0') * p[(k-2*i+1)/2];
    if(S[0]!='0' and S[k-1]!='0') ans += (S[0]-'1') * p[(k-1)/ 2];

    bool ok = true;
    for(int i=(k+1)/2-1;0<=i;i--){
        if(S[i]<S[k-i-1]) {ans++, ok = false; break;}
        if(S[i]!=S[k-i-1]) ok = false;
    }
    if(ok) ans++;

    cout << ans << endl;
    return 0;
}