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

ANS=0

REST=[]

for x in S:
    if x in {"3","5","7"}:
        ANS+=1
    else:
        REST.append(x)

ONE=0
NINE=0
LEN=len(REST)
USE=[0]*len(REST)

while True:
    while ONE<LEN and REST[ONE]!="1":
        ONE+=1

    NINE=max(ONE,NINE)

    while NINE<LEN and REST[NINE]!="9":
        NINE+=1

    if ONE==LEN or NINE==LEN:
        break
    else:
        ANS+=1
        USE[ONE]=1
        USE[NINE]=1
        ONE+=1
        NINE+=1

REST2=[]
for i in range(len(REST)):
    if USE[i]==0:
        REST2.append(REST[i])

REST=REST2

USE=[0]*len(REST)

flag=-1

for i in range(len(REST)):
    if flag==-1:
        if REST[i]=="1":
            flag=i

    else:
        if REST[i]=="1":
            USE[i]=1
            USE[flag]=1
            flag=-1
            ANS+=1

REST2=[]
for i in range(len(REST)):
    if USE[i]==0:
        REST2.append(REST[i])

REST=REST2

flag=-1
flag2=-1

for i in range(len(REST)):
    if flag==-1:
        if REST[i]=="9":
            flag=i

    else:
        if flag2==-1:
            if REST[i]=="9":
                flag2=i

        else:
            if REST[i]=="1":
                ANS+=1
                flag=-1
                flag2=-1

print(ANS)