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

ONE=0
NINE1=0
NINE2=0
LEN=len(REST)
USE=[0]*len(REST)

for i in range(len(REST)):
    while NINE1<LEN and REST[NINE1]!="9":
        NINE1+=1

    NINE2=NINE1+1

    while NINE2<LEN and REST[NINE2]!="9":
        NINE2+=1

    ONE=max(NINE2,ONE)

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

    if NINE1>=LEN or NINE2>=LEN or ONE>=LEN:
        break

    else:
        ANS+=1
        USE[ONE]=1
        USE[NINE1]=1
        USE[NINE2]=1
        ONE+=1
        NINE1=NINE2+1
        NINE2+=1
        
REST2=[]
for i in range(len(REST)):
    if USE[i]==0:
        REST2.append(REST[i])

REST=REST2

#print(ANS,REST)

ANS+=REST.count("1")//2
print(ANS)