#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    // 3, 5, 7
    // 19, 11, 991
    int N; string S; cin >> N >> S;
    int ans = 0, c1 = 0;
    string T = "";
    rep(i,N) {
        if(set<int>{3, 5, 7}.count(S[i] - '0')) ans++;
        if(S[i] == '1') c1++;
        if(S[i] == '9') {
            if(c1 == 0) T += S[i];
            else c1--, ans++;
        }
    }

    int c9 = 0;
    for(char c : T) if(c == '9') c9++;

    {
        int ANS = 0;
        for(int a1 = 0; a1 <= c1 / 2; a1++) {
            int r1 = c1 - a1 * 2;
            int a9 = min(c9 / 2, r1);
            ANS = max(ANS, a1 + a9);
        }
        ans += ANS;
    }
    cout << ans << endl;
}