#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include 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; using VVI = vector; using VL = vector; using VVL = vector; using VD = vector; using VVD = vector; using PII = pair; using PDD = pair; using PLL = pair; using VPII = vector; template using VT = vector; #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 templateinline bool chmax(T &a, T b){return (a < b ? a = b, true : false);} templateinline 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; } 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; //DEBUG(strbuf2) if (low >= stoll(strbuf2)) ++ans; } ++nn; if (nn == ma) break; } break; } } high = 1e9 + 10, low = 0; cout << ans << endl; }