#include<bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}


int solve(const string &S) {
  vector< int > one, nine;
  for(int i = 0; i < S.size(); i++) {
    if(S[i] == '1') one.emplace_back(i);
    else nine.emplace_back(i);
  }

  int ret = 0;
  for(int i = 0; i <= min(one.size(), nine.size() / 2); i++) {

    vector< int > used(S.size());

    bool ok = true;
    for(int j = 0; j < i; j++) {
      int k = j * 2 + 1;
      int l = (int) one.size() - i + j;
      used[nine[k - 1]] = true;
      used[nine[k]] = true;
      used[one[l]] = true;
      ok &= nine[k] < one[l];
    }
    if(!ok) continue;

    int o = 0, add = 0;
    for(int j = 0; j < S.size(); j++) {
      if(used[j]) continue;
      if(S[j] == '9') {
        if(o > 0) {
          --o;
          ++add;
        }
      } else {
        ++o;
      }
    }
    add += o / 2;
    chmax(ret, add + i);
  }
  return ret;
}


int main() {
  int N;
  string S;
  cin >> N >> S;
  int ret = 0;
  string T;
  for(auto &c : S) {
    if(c == '3' || c == '5' || c == '7') ++ret;
    else T += c;
  }
  cout << ret + solve(T) << endl;
}