#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto&(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

int main() {
  string s;
  cin >> s;
  int h = s[0] - '0';
  int len = SZ(s);
  int ans;
  if (h >= 5) {
    if (len % 2 == 1) {
      ans = (len - 1) / 2;
    } else {
      ans = len / 2 - 1;
    }
  } else {
    if (len % 2 == 1) {
      ans = (len + 1) / 2;
    } else {
      ans = len / 2;
    }
  }
  cout << ans << endl;
}