N = int(input())
lsS = list(map(int,list(input())))
cnt = 0
lsS1 = []
pS = set([3,5,7])
for i in range(N):
    if lsS[i] in pS:
        cnt += 1
    else:
        lsS1.append(lsS[i])
cn1 = 0
cn19 = 0
for s in lsS1:
    if s == 1:
        cn1 += 1
    else:
        if cn1 > 0:
            cnt += 1
            cn19 += 1
            cn1 -= 1
ls3 = []
cn19_ = cn19
for s in range(len(lsS1)-1,-1,-1):
    if cn19_>0 and (lsS1[s]==9):
        cn19_ -= 1
    else:
        ls3.append(lsS1[s])
ls3.reverse()
ls4 = []
cn19_ = cn19
for s in range(len(ls3)):
    if cn19_>0 and (ls3[s]==1):
        cn19_ -= 1
    else:
        ls4.append(ls3[s])
cn1 = 0
cn9 = 0
for i in range(len(ls4)):
    if ls4[i] == 9:
        cn9 += 1
    else:
        if cn9 >= 2:
            cnt += 1
            cn9 -= 2
        else:
            cn1 += 1
cnt += cn1//2
print(cnt)