N = int(input())
S = input()

one = 0
nine = 0
ans = 0

for i in S:
    if i == '3' or i == '5' or i == '7':
        ans += 1
    elif i == '1':
        one += 1
    elif i == '9':
        if one >= 1:
            one -= 1
            ans += 1
        else:
            nine += 1

tmp = min(nine // 2, one)
ans += tmp + (one - tmp) // 2
print(ans)