n = int(input())
s = input()
s = ''.join(s.split('3'))
s = ''.join(s.split('5'))
s = ''.join(s.split('7'))
ans = n - len(s)
c1 = 0
c9 = 0
for c in s:
    if c == '1':
        c1 += 1
    else:
        if c1 > 0:
            ans += 1
            c1 -= 1
        else:
            c9 += 1
while c1 >= 1 and c9 >= 2:
    ans += 1
    c1 -= 1
    c9 -= 2
ans += c1 // 2
print(ans)