#import algorithm, future, macros, math, sequtils, sets, strutils, tables
import algorithm, sugar, macros, math, sequtils, sets, strutils, tables

macro unpack(rhs: seq, cnt: static[int]): auto =
  let v = genSym(); result = quote do:(let `v` = `rhs`;())
  if NimMinor <= 17:
    for i in 0..<cnt: result[0][1].add(quote do:`v`[`i`])
  else:
    for i in 0..<cnt: result[1].add(quote do:`v`[`i`])

template input(T: typedesc, cnt: Natural = 1): untyped =
  let line = stdin.readLine.split(" ")
  when T is int:         line.map(parseInt).unpack(cnt)
  elif T is float:       line.map(parseFloat).unpack(cnt)
  elif T is string:      line.unpack(cnt)
  elif T is char:        line.mapIt(it[0]).unpack(cnt)
  elif T is seq[int]:    line.map(parseint)
  elif T is seq[float]:  line.map(parseFloat)
  elif T is seq[string]: line
  elif T is seq[char]:   line.mapIt(it[0])

proc `&=`(n: var int, m: int) = n = n and m
proc `|=`(n: var int, m: int) = n = n or m
proc `%=`(n: var int, m: int) = n = n mod m
proc `//=`(n: var int, m: int) = n = n div m
proc `<<=`(n: var int, m: int) = n = n shl m
proc `>>=`(n: var int, m: int) = n = n shr m
proc `<?=`(n: var SomeNumber, m: SomeNumber) = n = min(n, m)
proc `>?=`(n: var SomeNumber, m: SomeNumber) = n = max(n, m)
proc newSeq2[T](n1, n2: Natural): seq[seq[T]] = newSeqWith(n1, newSeq[T](n2))
proc newSeq3[T](n1, n2, n3: Natural): seq[seq[seq[T]]] = newSeqWith(n1, newSeqWith(n2, newSeq[T](n3)))

# -------------------------------------------------- #

var N = input(int)
var S = "#" & input(string) & "#"
var used = newseq[bool](N + 1)
var res = 0
var cnt1 = 0
var idx1 = newseq[int]()
var idx = 0
for i in 1 .. N:
  case S[i]:
    of '3', '5', '7':
      res += 1
      used[i] = true
    of '1':
      cnt1 += 1
      idx1.add(i)
    of '9':
      if cnt1 >= 1:
        used[idx1[idx]] = true
        used[i] = true
        cnt1 -= 1
        idx += 1
        res += 1
    else:
      discard
var cnt9 = 0
for i in 1 .. N:
  if used[i]:
    continue
  case S[i]:
    of '9':
      cnt9 += 1
    of '1':
      if cnt9 >= 2:
        used[i] = true
        cnt9 -= 2
        res += 1
    else:
      discard
cnt1 = 0
for i in 1 .. N:
  if S[i] == '1' and not used[i]:
    cnt1 += 1
res += cnt1 div 2
echo res