n = int(input())
s = input()
for i in "357":
    s = s.replace(i,"")
ans = n - len(s)
#11,19,991
one = nine = 0
for i,si in enumerate(s):
    if si == "1":
        one += 1
    else:
        if one:
            ans += 1
            one -= 1
        else:
            nine += 1
c = min(one,nine//2)
ans += c
one -= c
nine -= 2*c
ans += one//2
print(ans)