#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); i++)
#define repr(i, n) for (int i = (n) - 1; i >= 0; i--)
#define rep2(i, l, r) for (int i = (l); i < (r); i++)
#define rep2r(i, l, r) for (int i = (r) - 1; i >= (l); i--)
#define range(a) a.begin(), a.end()

using namespace std;
using ll = long long;

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  int n; cin >> n;
  string s; cin >> s;
  int ans = 0;
  int cnt[10] = {};
  // 991991
  // 991
  // 19
  // 11
  vector<bool> removed(n);
  int i = 0;
  int j = n - 1;
  while (i < j) {
    while (i < n && s[i] != '1') i++;
    while (j >= 0 && s[j] != '9') j--;
    if (i < j) {
      ans++;
      removed[i] = true;
      removed[j] = true;
      i++;
      j--;
    }
  }
  rep(i, n) if (!removed[i]) {
    if (s[i] == '1' && cnt[9] > 2) {
      ans++;
      cnt[9] -= 2;
    } else {
      cnt[s[i] - '0']++;
    }
  }
  ans += cnt[1] / 2;
  ans += cnt[3];
  ans += cnt[5];
  ans += cnt[7];
  cout << ans << endl;
}