n=int(input())
s=input()
ans=0
c=""
for i in s:
    if i == "3" or i == "5" or i == "7":
        ans+=1
    else:
        c+=i
end9=-1
cnt=0
for i in range(len(c))[::-1]:
    if c[i]=="9" and end9==-1:end9=i;cnt+=1
    if c[i]=="9":cnt+=1
if end9 == -1:
    print(ans+len(c)//2)
else:
    ans+=max(min(end9+1-cnt,cnt),0)
    print(ans+(len(c)-end9-1)//2)