#include 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 P; typedef pair PL; const ll mod = 1000000007; const ll INF = 1000000099; const ll LINF = (ll)(1e18 + 99); vector dx = {-1, 1, 0, 0}, dy = {0, 0, -1, 1}; template T gcd(T a, T b) { return b ? gcd(b, a % b) : a; } template 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 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; }