#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using db = double;
using ld = long double;
#define fs first
#define sc second
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define eb emplace_back
#define all(v) (v).begin(), (v).end()
#define siz(v) (ll)(v).size()
#define rep(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define repr(i, a, n) for (ll i = n - 1; (ll)a <= i; i--)
#define lb lower_bound
#define ub upper_bound
typedef pair<int, int> P;
typedef pair<ll, ll> PL;
const ll mod = 1000000007;
const ll INF = 1000000099;
const ll LINF = (ll)(1e18 + 99);
vector<ll> dx = {-1, 1, 0, 0}, dy = {0, 0, -1, 1};
template <typename T>
T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template <typename T>
T mpow(T a, T n)
{
    T res = 1;
    for (; n; n >>= 1)
    {
        if (n & 1)
            res = res * a;
        a = a * a;
    }
    return res;
}

//cin.tie(0);ios::sync_with_stdio(false);

signed main()
{
    string s, t;
    int n, ans = 0;
    cin >> n >> t;
    rep(i, 0, n)
    {
        if (t[i] == '3' || t[i] == '5' || t[i] == '7')
        {
            ans++;
        }
        else
        {
            s.pb(t[i]);
        }
    }

    stack<int> st;
    rep(i, 0, siz(s))
    {
        if (s[i] == '1')
            st.push(1);
        else
        {
            if (!st.empty() && st.top() == 1)
            {
                ans++;
                st.pop();
            }
            else
            {
                st.push(9);
            }
        }
    }

    int cnt = 0;
    while (!st.empty() && st.top() == 1)
    {
        cnt++;
        st.pop();
    }

    int cnt2 = 0;
    while (!st.empty() && st.top() == 9)
    {
        st.pop();
        cnt2++;
    }

    ans+=min(cnt,cnt2/2);
    cnt-=min(cnt,cnt2/2);
    ans+=cnt/2;

    cout << ans << endl;
}