#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
#define ce(x) cerr << (x) << "\n"
#define cesp(x) cerr << (x) << " "
#define pb push_back
#define mp make_pair
#define Would
#define you
#define please

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	int N;
	cin >> N;
	string S;
	cin >> S;

	int kotae = 0;
	int kyuu = 0;
	for (int i = N - 1; i >= 0; i--) {
		if (S[i] == '3' || S[i] == '5' || S[i] == '7') kotae++;
		else if (S[i] == '9') kyuu++;
		else {
			if (kyuu) {
				kyuu--;
				kotae++;
				S[i] = '.';
			}
		}
	}
	int kazu = 0;
	rep(i, N) {
		ce(S[i]);
		if (S[i] == '1') kazu++;
	}
	co(kotae + kazu / 2);

	Would you please return 0;
}