#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define rep(i, n) for(ll (i) = 0; (i) < (n); (i)++)
#define rep1(i, n) for(ll (i) = 1; (i) <= (n); (i)++)
#define rrep(i, n) for(ll (i) = (n) - 1; (i) >= 0; (i)--)
#define rrep1(i, n) for(ll (i) = (n); (i) >= 1; (i)--)
const ll INF = 1145141919;
const ll MOD = 1000000007;
template<class T> void chmax(T &a, const T &b){if(a < b){a = b;}}
template<class T> void chmin(T &a, const T &b){if(a > b){a = b;}}

char s[202020];
ll flg[202020];

int main(){

    ll N, ans = 0;
    cin >> N;
    rep1(i, N){
        cin >> s[i];
        if(s[i] == '3')ans++;
        if(s[i] == '5')ans++;
        if(s[i] == '7')ans++;        
    }
    ll one = 0;
    ll nine = 0;
    rrep1(i, N){
        if(s[i] == '9')nine++;
        if(s[i] == '1'){
            if(nine){
                nine--;
                ans++;
            }
            else one++;
        }
    }
    ans += one / 2;
    cout << ans << endl;
    
    return 0;
}