#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i,n) for(int (i)=((n)-1);(i)>=0;(i)--)
#define itn int
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
const long long INF = 1LL << 60;
const int MOD = 1000000007;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
signed main(void){
    int n; cin>>n;
    string s; cin>>s;
    int ans = 0;
    int one = -1, nine = -1;
    rep(i,n){
        if(s[i]=='3'|| s[i] == '5'|| s[i] == '7'){
            ans++;
            s[i] = '?';
        }
        if(s[i] == '1') one = i;
        if(s[i] == '9') {
            nine = i;
            if(one != -1) {
                s[one] = s[nine] = '?';
                one = -1; nine = -1;
                ans++;
            }
        }
    }
    bool update = true;
    while(update){
        update = false;
        one = -1; nine = -1;
        rep(i,n){
            if(s[i] == '1') one = i;
            if(s[i] == '9') {
                nine = i;
                if(one != -1) {
                    s[one] = s[nine] = '?';
                    one = -1; nine = -1;
                    ans++;
                    update = true;
                }
            }
        }
    }
    int c1=0, c9 = 0;
    rep(i,n){
        if(s[i] == '1') c1++;
        else if(s[i] == '9') c9++;
    }
    ans += min(c9/2, c1);
    cout<<ans<<endl;
}