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

int main() {
  ios::sync_with_stdio(false);

  int N;
  cin >> N;

  string S;
  cin >> S;

  vector<int> cnt(10);
  for (char c : S) ++cnt[c - '0'];

  int ans = 0;
  for (int p : { 3, 5, 7 }) ans += cnt[p];

  vector<int> onepos;
  for (int i = 0; i < N; ++i) {
    if (S[i] == '1') {
      onepos.push_back(i);
    }
  }

  for (int i = N - 1; ~i; --i) {
    if (S[i] != '9') continue;
    while (onepos.size() && i < onepos.back()) {
      onepos.pop_back();
    }
    if (onepos.size()) {
      assert(onepos.back() < i);
      onepos.pop_back();
      --cnt[9];
      --cnt[1];
      ++ans;
    }
  }

  ans += min(cnt[9] / 2, cnt[1]);
  cnt[1] -= min(cnt[9] / 2, cnt[1]);
  ans += cnt[1] / 2;

  cout << ans << endl;

  return 0;
}