MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

def main():
    n = int(input())
    s = input()
    stack = [-1]
    cnt9 = 0
    ans = 0
    for i in range(n):
        if s[i] in '357':
            ans += 1
        elif s[i] == '9':
            if stack[-1] == '1':
                ans += 1
                stack.pop()
            else:
                cnt9 += 1
        else:
            stack.append(s[i])
    
    cnt1 = stack.count('1')
    while cnt1 > 0 and cnt9 > 0:
        cnt1 -= 1
        cnt9 -= 2
        ans += 1
    ans += cnt1//2
    print(ans)

if __name__ =='__main__':
    main()