#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define REP1(i, n) for(int i=1; i<=(n); i++)
#define RREP1(i, n) for(int i=(n); i>=1; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define out(...) printf(__VA_ARGS__)

int dxy[] = {0, 1, 0, -1, 0};

void solve();
signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}

/*================================*/
#if DEBUG
#define SIZE 100
#else
#define SIZE 123450
#endif

int N,M,Q;
string S;

void solve() {
    cin>>N>>S;

    int ans = 0;
    ans += count(ALL(S), '3');
    ans += count(ALL(S), '5');
    ans += count(ALL(S), '7');
    vector<int> T;
    for (char c:S) {
        int i = c-'0';
        if (i == 1 || i == 9) T.push_back(i);
    }
    while(T.size()) {
        auto it1 = find(ALL(T), 1);
        if (it1 == T.end()) break;
        auto it9 = find(it1, T.end(), 9);
        if (it9 == T.end()) break;
        ans++;
        T.erase(it9);
        T.erase(it1);
    }
    while(T.size()) {
        auto it90 = find(T.begin(), T.end(), 9);
        if (it90 == T.end()) break;
        auto it91 = find(it90+1, T.end(), 9);
        if (it91 == T.end()) break;
        auto it1 = find(it91, T.end(), 1);
        if (it1 == T.end()) break;
        ans++;
        T.erase(it1);
        T.erase(it91);
        T.erase(it90);
    }
    ans += count(ALL(T), 1)/2;
    
    cout << ans << endl;
}