#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <complex>
using namespace std;

#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)

typedef long long ll;
typedef uint64_t ull;
typedef pair<int, int> P;
typedef pair<int, double> Pid;
typedef pair<double, int> Pdi;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<int, pair<int, int>> PP;
typedef pair<P, int> PPi;
constexpr double PI = 3.1415926535897932;   // acos(-1)
constexpr double EPS = 1e-9;
constexpr int INF = 1001001001;
constexpr int mod = 1e+9 + 7;
// constexpr int mod = 998244353;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n;
    string s;
    cin >> n >> s;
    int ans = 0;
    queue<int> One;
    set<int> Nine;
    for(int i = n - 1; i >= 0; --i){
        if(s[i] == '3' || s[i] == '5' || s[i] == '7')   ++ans;
        else if(s[i] == '9')    Nine.insert(i);
        else    One.push(i);
    }
    int OneCnt = 0, NineCnt = 0;
    int left = 0;
    while(!One.empty()){
        int pos = One.front();
        One.pop();
        auto it = Nine.lower_bound(pos);
        if(it != Nine.end()){
            ++ans;
            Nine.erase(*it);
        }
        else{
            ++OneCnt;
            left = pos;
        }
    }
    for(auto it : Nine){
        if(it < left)   ++NineCnt;
    }
    ans += min(NineCnt / 2, OneCnt);

    cout << ans << endl;
}