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
for s in lsS1:
    if s == 1:
        cn1 += 1
    else:
        if cn1 > 0:
            cnt += 1
            cn1 -= 1
cnt += cn1//2
print(cnt)