#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<bitset>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<deque>
#include<list>
#include<iomanip>
#include<cmath>
#include<cstring>
#include<functional>
using namespace std;

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define INF 2e9
#define MOD 1000000007
//#define MOD 998244353
#define LINF (long long)4e18
#define jck 3.141592

using ll = long long;
using Pi = pair<int,int>;
using Pl = pair<ll,ll>;


int main(){
    int N; cin >> N;
    string S; cin >> S;
    int ans = 0;
    int now1 = 0;
    int now9 = 0;
    rep(i,N){
        if(S[i] == '3' || S[i] == '5' || S[i] == '7') ans++;
        if(S[i] == '1'){
            now1++;
        }
        if(S[i] == '9'){
            now9++;
            if(now1 > 0){
                now1--;
                now9--;
                ans++;
            }
        }
    }
    ans += min(now9/2,now1);
    now1 -= min(now9/2,now1);
    if(now1 > 0) ans += now1/2;
    cout << ans << endl;
}