import times, strutils, sequtils, math, algorithm, tables, sets, lists, intsets
import critbits, future, strformat, deques
template `max=`(x,y) = x = max(x,y)
template `min=`(x,y) = x = min(x,y)
template `mod=`(x,y) = x = x mod y
template scan2 = (scan(), scan())
template scan3 = (scan(), scan())
let read* = iterator: string {.closure.} =
    while true: (for s in stdin.readLine.split: yield s)
proc scan(): int = read().parseInt
proc scanf(): float = read().parseFloat
proc toInt(c:char): int =
    return int(c) - int('0')



proc solve():int=
  var
    n = scan()
    s = read()
    one = 0
    three = s.count('3')
    five = s.count('5')
    seven = s.count('7')
    nine = 0
  # 3,5,7:他と組み合わせて除去して得になる瞬間がないので単体で除去する
  # 11:素数
  # 19:素数
  # 9は
  # 91:7の倍数
  # 911..:素数かもしれないけど11を除去するのでいい
  # 991:素数。
  for c in s:
    if c == '1':
      one+=1
    if c == '9':
      nine+=1
      if one>0:
        one-=1
        nine-=1
        result+=1

  while nine>=2 and one > 0:
    result+=1
    nine-=2
    one-=1
  return result+three+five+seven+one.div(2)
  
echo solve()