#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(i64 (i) = (s);(i) < (e);(i)++)
#define all(x) x.begin(),x.end()
#define let auto const

template<typename... Types>
struct dynarr: std::vector<Types...> {
  using std::vector<Types...>::vector;
  using size_type = typename std::vector<Types...>::size_type;
  auto&& operator[](size_type i) { return this->at(i); }
  auto&& operator[](size_type i) const { return this->at(i); }
};

int main() {
  i64 N;
  string s;
  cin >> N >> s;
  string t = "";
  i64 ans = 0;
  for(int i = 0;i < N;i++) {
    if(s[i] == '3' || s[i] == '5' || s[i] == '7') {
      ans++;
    }
    else {
      t.push_back(s[i]);
    }
  }
  i64 one = 0;
  for(int i = 0;i < t.size();i++) {
    if(t[i] == '1') {
      one++;
    }
    else {
      if(one > 0) {
        ans++;
        one--;
      }
    }
  }
  cout << ans + one / 2 << endl;
}