#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int n;
  string s;
  cin>>n>>s;
  Int ans=0,one=0;
  for(char c:s){
    if(c=='1') one++;
    if(c=='9') if(one) ans++,one--;
    if(c=='3'||c=='5'||c=='7') ans++;
  }
  ans+=one/2;
  cout<<ans<<endl;
  return 0;
}