#include <bits/stdc++.h>
using namespace std;
#define modulo 1000000007
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 100000000



int main(){
	
	int N;
	cin>>N;
	string S;
	cin>>S;
	
	string T = "";
	int ans = 0;
	
	for(int i=0;i<N;i++){
		if(S[i]=='3'||S[i]=='5'||S[i]=='7'){
			ans++;
			continue;
		}
		T += S[i];
	}
	
	reverse(T.begin(),T.end());
	
	int cnt9=0,cnt1=0;
	for(int i=0;i<T.size();i++){
		if(T[i]=='9'){
			cnt9++;
		}
		else{
			if(cnt9>0){
				cnt9--;
				ans++;
			}
			else{
				cnt1++;
			}
		}
	}
	
	ans += min(cnt1,cnt9/2);
	cnt1 -= cnt9/2;
	
	if(cnt1>0)ans += cnt1/2;
	
	
	
	cout<<ans<<endl;
	
    return 0;
}