#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define all(c) c.begin(),c.end()
#define pb push_back
#define fs first
#define sc second
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
using namespace std;
template<class S,class T> ostream& operator<<(ostream& o,const pair<S,T> &p){
	return o<<"("<<p.fs<<","<<p.sc<<")";
}
template<class T> ostream& operator<<(ostream& o,const vector<T> &vc){
	o<<"{";
	for(const T& v:vc) o<<v<<",";
	o<<"}";
	return o;
}
using ll = long long;
template<class T> using V = vector<T>;
template<class T> using VV = vector<vector<T>>;
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n-1); }

#ifdef LOCAL
#define show(x) cerr << "LINE" << __LINE__ << " : " << #x << " = " << (x) << endl
#else
#define show(x) true
#endif

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);		//DON'T USE scanf/printf/puts !!
	cout << fixed << setprecision(20);
	
	int N;string s; cin >> N >> s;
	int res = 0;
	V<int> v;
	for(char c: s){
		int d = c-'0';
		if(d == 3 || d == 5 || d == 7) res++;
		else v.pb(d);
	}
	{	//19
		V<int> nv;
		for(int x: v){
			if(x == 9 && !nv.empty() && nv.back() == 1){
				nv.pop_back();
				res++;
			}else{
				nv.pb(x);
			}
		}
		v = nv;
	}
	int one = 0, nine = 0;
	for(int x: v){
		if(x == 1) one++;
		else nine++;
	}
	int a = min(nine/2,one);
	res += a;
	one -= a;
	res += one/2;
	cout << res << endl;
}