#include using namespace std; #define FOR(i, m, n) for(int (i) = (m); (i) < (n); (i)++) #define REP(i, n) FOR(i, 0, n) #define co(n) cout << (n) << endl #define cosp(n) cout << (n) << ' ' #define all(s) (s).begin(), (s).end() #define pb push_back #define mp make_pair #define fs first #define sc second typedef long long ll; typedef pair P; const ll INF = 2e9; const ll MOD = 1e9+7; //const ll MOD 998244353; int main(void){ int n; cin >> n; string arr; cin >> arr; int ans = 0, num1 = 0, num9 = 0; REP(i, n){ if(arr[i] == '3' || arr[i] == '7') ans++; else if(arr[i] == '1') num1++; else{ if(num1){ ans++; num1--; }else num9++; } } while(num1 && num9 >= 2){ ans++; num1--; num9 -= 2; } ans += num1/2; co(ans); return 0; }