N = int(input())
S = list(input())
ans = 0
nine = 0
one = 0
for i in S:
    if i in '357':
        ans += 1
    if i == '9':
        if one >= 1:
            ans += 1
            one -= 1
        else:
            nine += 1
    if i == '1':
        if nine >= 2:
            ans += 1
            nine -= 2
        else:
            one += 1
ans += one//2
print(ans)