#include<bits/stdc++.h>
using namespace std;
using ll = long long;
template<class T> using vt = vector<T>;
template<class T> using vvt = vector<vt<T>>;
template<class T> using ttt = tuple<T,T>;
using tii = tuple<int,int>;
using tiii = tuple<int,int,int>;
using vi = vector<int>;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define pb push_back
#define mt make_tuple
#define ALL(a) (a).begin(),(a).end()
#define FST first
#define SEC second
#define DEB cerr<<"!"<<endl
#define SHOW(a,b) cerr<<(a)<<" "<<(b)<<endl
const int INF = (INT_MAX/2);
const ll LLINF = (LLONG_MAX/2);
const double eps = 1e-8;
const ll DIV =1e9+7;
//const double PI = M_PI;
inline ll pow(ll x,ll n,ll m){ll r=1;while(n>0){if((n&1)==1)r=r*x%m;x=x*x%m;n>>=1;}return r%m;}
inline ll lcm(ll d1, ll d2){return d1 / __gcd(d1, d2) * d2;}
#define chmax(a,b) a=max(a,b)

/*Coding Space*/
int n;
string s;
int c1,c3,c5,c7;
int main(){
  cin >> n >> s;
  int ans = 0;
  rep(i,n){
    if(s[i] == '1') c1++;
    else if(s[i] == '3') c3++;
    else if(s[i] == '5') c5++;
    else if(s[i] == '7') c7++;
    else if(s[i] == '9'){
      if(c1){ans++; c1--;}
    }
  }
  ans += c5+c7+c3;
  ans += c1/2;
  cout << ans << endl;
}