import sequtils, strutils, tables

let
  n = parseInt readLine stdin
  abcs = concat n.newSeqWith stdin.readLine.split.map parseInt
var
  cntT = abcs.toCountTable
  cnt = [0, 0]

for i in 1 .. 10:
  cnt = [cnt[0] + cntT[i] div 2, cnt[1] + cntT[i] mod 2]

echo cnt[0] + cnt[1] div 4