#include<bits/stdc++.h>
using namespace std;
#define fs first
#define sc second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ALL(A) A.begin(),A.end()
#define RALL(A) A.rbegin(),A.rend()
typedef long long LL;
typedef pair<LL,LL> P;
LL mod=1000000007;
LL LINF=1LL<<60;
const int INF=1<<30;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};



int main(){
    int n;cin >> n;
    string s;
    cin >> s;
    int ans = 0;
    int a=0,b=0;
    for (int i = 0; i < n; i++) {
        if(s[i]=='3'||s[i]=='5'||s[i]=='7'){
            ans++;
        }
        else if(s[i]=='9'){
            if(b){
                ans++;
                b--;
            }
            else a++;
        }
        else if(s[i]=='1'){
            if(a>=2){
                a-=2;
                ans++;
            }
            else b++;
        }
    }
    cout << ans << endl;
    return 0;
}