N = int(input())
S = list(input())
S = list(map(int, S))
ans = S.count(3) + S.count(5) + S.count(7)
one = 0
for s in S:
    if s == 1:
        one += 1
    elif s == 9:
        if one:
            ans += 1
            one -= 1
print(ans + one//2)