#include <bits/stdc++.h>

#ifndef LOCAL_
#define fprintf if( false ) fprintf
#endif // LOCAL_
#define dumpi(x1) fprintf(stderr, "#%s.%d (%s) = (%d)\n", __func__, __LINE__, #x1, x1);
#define dumpii(x1, x2) fprintf(stderr, "#%s.%d (%s, %s) = (%d, %d)\n", __func__, __LINE__, #x1, #x2, x1, x2);
#define dumpiii(x1, x2, x3) fprintf(stderr, "#%s.%d (%s, %s, %s) = (%d, %d, %d)\n", __func__, __LINE__, #x1, #x2, #x3, x1, x2, x3);
#define dumpl(x1) fprintf(stderr, "#%s.%d (%s) = (%ld)\n", __func__, __LINE__, #x1, x1);
#define dumpll(x1, x2) fprintf(stderr, "#%s.%d (%s, %s) = (%ld, %ld)\n", __func__, __LINE__, #x1, #x2, x1, x2);
#define dumpd(x1) fprintf(stderr, "#%s.%d (%s) = (%lf)\n", __func__, __LINE__, #x1, x1);
#define dumpdd(x1, x2) fprintf(stderr, "#%s.%d (%s, %s) = (%lf, %lf)\n", __func__, __LINE__, #x1, #x2, x1, x2);

template<typename T>
T pow(T x, long n) {
  T res = 1;
  T p = x;
  while( n != 0 ) {
    if( n & 0x01 ) res *= p;
    p *= p;
    n >>= 1;
  }
  return res;
}

// void greedy(long x) {
//   long t = 1000000001;
//   for(long i = t; i <= x; i+=t) {
//     if( 
//   }
  
// }

long f(std::string str, int b, int n) {
  if( n == 0 ) return 0;
  if( n == 1 ) return str[b] - '0' + (b == 0 ? 0 : 1);
  long res = pow(10, n - 1) * std::max(0, str[b] - '0' - (b == 0 ? 1 : -1)) + f(str, b + 1, n - 1);
  dumpl(res);
  return res;
}

int main() {
  long dp[32] = {};
  // dp[1] = 9;
  // dp[2] = 9;
  // dp[3] = 9 * 10;
  // dp[4] = 9 * 10;
  // dp[5] = 9 * 10 * 10;
  for(int i = 0; i < 32; ++i) {
    dp[i] = 9 * pow(10, (i-1)/2);
  }
  
  std::string str;
  std::cin >> str;
  long t = std::stol(str);
  if( t < 1000000001 ) {
    puts("0");
    return 0;
  }
  long low  = t % 1000000000;
  long high = t / 1000000000;
  long m = (int)str.size() - 9;
  long x = f(str, 0, (m + 1) / 2);
  dumpl(m);
  // long x = 0;
  for(int i = 1; i < m; ++i) {
    x += dp[i];
  }
  // for(int i = 1; i < m; ++i) {
  //   x += dp[i];
  // }
  // x += (str[0] - '0') * pow(10, std::max((long)0, (m - 1)/2));
  long res = x;
  if( high > low ) res = std::max(long(0), res - 1);
  printf("%ld\n", res);
  
  return 0;
}