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

one = 0
nine = 0
ans = 0
for s in S:
    if s == "1":
        if nine >= 2:
            nine -= 2
            ans += 1
        else:
            one += 1
    elif s == "9":
        if one:
            one -= 1
            ans += 1
        else:
            nine += 1
    else:
        ans += 1
ans += one // 2
print(ans)