#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);
    }
    int i1=0, i90=0, i91=0;
    while(T.size()) {
        while(i1<T.size()) {
            if (T[i1] == 1) break;
            i1++;
        }
        if (i1 >= T.size()) break;
        i90 = max(i90, i1+1);
        while(i90<T.size()) {
            if (T[i90] == 9) break;
            i90++;
        }
        if (i90 >= T.size()) break;
        ans++;
        T[i1]=T[i90]=0;
    }
    i1 = i90 = 0;
    while(T.size()) {
        while(i90<T.size()) {
            if (T[i90] == 9) break;
            i90++;
        }
        if (i90 >= T.size()) break;
        i91 = i90 + 1;
        while(i91<T.size()) {
            if (T[i91] == 9) break;
            i91++;
        }
        if (i91 >= T.size()) break;
        i1 = max(i1, i91);
        while(i1<T.size()) {
            if (T[i1] == 1) break;
            i1++;
        }
        if (i1 >= T.size()) break;
        ans++;
        T[i1]=T[i90]=T[i91]=0;
    }
    ans += count(ALL(T), 1)/2;
    
    cout << ans << endl;
}