N=int(input())
S=input()
mp=[0 for i in range(10)]

for c in S:
    mp[ord(c)-ord('0')]+=1

S=S[::-1]
res=mp[3]+mp[5]+mp[7]

c1=0
c9=0
for c in S:
    if c=='1':
        if c9>0:
            c9-=1
            res+=1
        else:
            c1+=1
    if c=='9':
        c9+=1
res+=c1//2

print(res)