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

L = []
ans = 0
for i in S:
  if i == "3" or i == "5" or i == "7":
    ans += 1
  else:
    L.append(int(i))

o = 0
n = 0
for i in L:
  if i == 1:
    o += 1
  if i == 9:
    n += 1
    if o != 0:
      o -= 1
      n -= 1
      ans += 1

ans += min(n // 2, o)
o -= n // 2
if o > 0:
  ans += o // 2
print(ans)