#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,
      ans = 0;
  rep(i, N) {
    if (S[i] == '3' || S[i] == '5' || S[i] == '7')
      ++ans;
    if (S[i] == '1')
      ++one;
  }
  cout << ans + one / 2 << endl;
  return 0;
}