from fractions import gcd
from datetime import date, timedelta
from heapq import*
import math
from collections import defaultdict, Counter, deque
import sys
from bisect import *
import itertools
import copy
sys.setrecursionlimit(10 ** 7)
MOD = 10 ** 9 + 7


def main():
    n = int(input())
    s = list(input())
    s2 = []
    ans = 0
    for i in range(n):
        if s[i] == "3" or s[i] == "5" or s[i] == "7":
            ans += 1
        else:
            s2.append(s[i])
    
    one, nine = 0, 0
    for i in range(len(s2)):
        if s2[i] == "1":
            one+=1
        elif s2[i] == "9":
            if one > 0:
                ans += 1
                one -= 1
    ans += (one // 2)
    print(ans)


    
            
    

    

    

if __name__ == '__main__':
    main()