import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
import time
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines
write=sys.stdout.write

N=int(readline())
ans=0
S=[]
for s in readline().rstrip():
    if s in "357":
        ans+=1
    else:
        S.append(s)
    if len(S)>=2 and S[-2:]==["1","9"]:
        S.pop()
        S.pop()
        ans+=1
cnt9,cnt1=S.count("9"),S.count("1")
cnt=min(cnt9//2,cnt1)
ans+=cnt
cnt9-=2*cnt
cnt1-=cnt
ans+=cnt1//2
print(ans)