/*** author: yuji9511 ***/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> lpair;
const ll MOD = 1e9+7;
const ll INF = 1e18;
#define rep(i,m,n) for(ll i=(m);i<(n);i++)
#define rrep(i,m,n) for(ll i=(m);i>=(n);i--)
#define printa(x,n) for(ll i=0;i<n;i++){cout<<(x[i])<<" \n"[i==n-1];};
void print() {}
template <class H,class... T>
void print(H&& h, T&&... t){cout<<h<<" \n"[sizeof...(t)==0];print(forward<T>(t)...);}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	ll N;
	string S;
	cin >> N >> S;
	ll ans = 0;
	string T = "";
	rep(i,0,N){
		if(S[i] == '3' || S[i] == '5' || S[i] == '7'){
			ans++;
		}else{
			T += S[i];
		}
	}
	ll sz = T.size();
	ll cnt1 = 0;
	rep(i,0,sz){
		if(T[i] == '1'){
			cnt1++;
		}else{
			if(cnt1 > 0){
				ans++;
				cnt1--;
			}
		}
	}
	ans += cnt1/2;
	print(ans);


	

}