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
        7
        11
        19
        59
        991
        9551
        555555555551
      */
      
      auto oks = new bool[1 << N];
      foreach (p; 0 .. 1 << N) {
        string s;
        foreach (i; 0 .. N) {
          if (p & 1 << i) {
            s ~= S[i];
          }
        }
        oks[p] = (s == "3" || s == "7" || s == "11" || s == "19" || s == "59" || s == "991" || s == "9551" || s == "555555555551");
      }
      auto dp = new int[1 << N];
      foreach (p; 1 .. 1 << N) {
        for (int q = p; q; --q &= p) {
          if (oks[q]) {
            chmax(dp[p], dp[p ^ q] + 1);
          }
        }
      }
      writeln(dp[$ - 1]);
      continue;
      
      int ans;
      string s;
      foreach (c; S) {
        if (c == '3' || c == '7') {
          ++ans;
        } else {
          s ~= c;
        }
      }
      switch (s) {
        case "11":
        case "19":
        case "59":
        case "991":
        case "9551":
        case "555555555551":
          writeln(ans + 1);
          break;
        default:
          // tasukete~
          import core.stdc.stdlib;
          exit(1);
      }
    }
  } catch (EOFException e) {
  }
}