#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;

//template
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(a);i>(b);i--)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll; typedef pair<ll, ll> P;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename A,size_t N,typename T>void Fill(A(&array)[N],const T &val){fill((T*)array, (T*)(array+N), val);}
const int inf = 0x3fffffff; const ll INF = 0x3fffffffffffffff;
//template end



int main(){
    int n; scanf("%d",&n);
    string s; cin>>s;
    int ans=0,one=0,nine=0;
    rep(i,0,n){
        if(s[i]=='3'||s[i]=='5'||s[i]=='7')ans++;
        if(s[i]=='1')one++;
        if(s[i]=='9'){
            if(one)one--,ans++;
            else nine++;
        }
    }
    int add=min(one,nine/2);
    ans+=add; one-=add;
    printf("%d\n",ans+one/2);
    return 0;
}