#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;

int main(){
  int n;
  cin >> n;
  string s;
  cin >> s;
  int res = 0,cnt1 = 0,cnt9 = 0;
  rep(i,n) {
    if (s[i] == '3' || s[i] == '5' || s[i] == '7') res++;
    if (s[i] == '1') cnt1++;
    if (s[i] == '9') {
      cnt9++;
      if (cnt1 > 0) cnt1--,cnt9--,res++;
    }
  } 
  int cnt991 = min(cnt9/2, cnt1);
  res += cnt991;
  res += (cnt1 - cnt991)/2;
  cout << res << endl;
  return 0;
}