#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	int n;
	cin>>n;
	string s;
	cin>>s;
	int ans=0;
	for(int i=0; i<n; i++){
		if(s[i]=='3' || s[i]=='7' || s[i]=='5') ans++;
	}
	queue<int> que;
	bool used[200020]={};
	for(int i=n-1; i>=0; i--){
		if(s[i]=='9') que.push(i);
		if(s[i]=='1'){
			if(!que.empty()){
				used[que.front()]=1;
				used[i]=1;
				que.pop();
				ans++;
			}
		}
	}
	int cnt9=0;
	for(int i=0; i<n; i++){
		if(s[i]=='9' && !used[i]) cnt9++;
		if(s[i]=='1' && !used[i]){
			if(cnt9>=2){
				cnt9-=2;
				ans++;
				used[i]=1;
			}
		}
	}
	int cnt=0;
	for(int i=0; i<n; i++){
		if(s[i]=='1' && !used[i]) cnt++;
	}
	ans+=cnt/2;
	cout<<ans<<endl;
    return 0;
}