не работает программа на lua

a={}
n=10
c=0
for i=1,n do
  table.insert(a,tonumber(io.read()))
end
for i=1,(#a-1) do
  for g=1,#a do
    if a[g]==a[i] then c=c+1 end
  end
end
f=io.open('out','w')
io.write(f,tostring(c)..'\n')
io.close(f)

lua: file.lua:13: bad argument #1 to 'write' (string expected, got userdata)
stack traceback:
	[C]: in function 'write'
	file.lua:13: in main chunk
	[C]: in ?