#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N;
string S;

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>S;
	int L1=0,L9=0,ret=0;
	FORR(c,S) {
		if(c=='3' || c=='5' || c=='7') ret++;
		else if(c=='1') {
			L1++;
		}
		else if(c=='9') {
			if(L1) {
				L1--;
				ret++;
			}
			else {
				L9++;
			}
		}
	}
	
	x=min(L9/2,L1);
	ret+=x+(L1-x)/2;
	cout<<ret<<endl;
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}