#include <iostream>
using namespace std;
#define rep(i,n) for(int i=0,_i=(n);i<_i;++i)

int main() {
  int N; string S;
  cin >> N >> S;
  int one = 0,
      nine = 0,
      ans = 0;
  rep(i, N) {
    if (S[i] == '3' || S[i] == '5' || S[i] == '7')
      ++ans;
    if (S[i] == '1')
      ++one;
    if (S[i] == '9') {
      ++nine;
      if (one > 0) {
        --one, --nine;
        ++ans;
      }
    }
  }
  if (nine >= one * 2)
    ans += one;
  else {
    ans += nine / 2;
    one -= nine / 2;
    ans += one / 2;
  }
  cout << ans << endl;
  return 0;
}