import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.complex, std.container, std.math, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 10^^9;

void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const S = readToken();
      
      /*
        3
        5 <- !!!
        7
        11
        19
        991
      */
      
      int ans;
      ans += S.count('3');
      ans += S.count('5');
      ans += S.count('7');
      
      auto s9 = new RedBlackTree!int;
      foreach (i; 0 .. N) {
        if (S[i] == '9') {
          s9.insert(i);
        }
      }
      debug {
        writeln("s9 = ", s9);
      }
      
      auto used1 = new bool[N];
      foreach_reverse (i; 0 .. N) {
        if (S[i] == '1' && !used1[i]) {
          auto r = s9.upperBound(i);
          if (!r.empty) {
            debug {
              writeln("19 ", i, " ", r.back);
            }
            ++ans;
            used1[i] = true;
            s9.removeKey(r.back);
          }
        }
      }
      foreach (i; 0 .. N) {
        if (S[i] == '1' && !used1[i]) {
          auto r = s9.lowerBound(i);
          if (!r.empty) {
            const j = r.front;
            r = r.dropOne;
            if (!r.empty) {
              debug {
                writeln("991 ", j, " ", r.front, " ", i);
              }
              ++ans;
              used1[i] = true;
              s9.removeKey(r.front);
              s9.removeKey(j);
            }
          }
        }
      }
      
      int cnt1;
      foreach (i; 0 .. N) {
        if (S[i] == '1' && !used1[i]) {
          ++cnt1;
        }
      }
      ans += cnt1 / 2;
      
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}