local mce, mfl, msq, mmi, mma = math.ceil, math.floor, math.sqrt, math.min, math.max
local function lower_bound(ary, x, num)
  if num == 0 then return 1 end
  if(x <= ary[1].val) then return 1 end
  if(ary[num].val < x) then return num + 1 end
  local min, max = 1, num
  while(1 < max - min) do
    local mid = mfl((min + max) / 2)
    if(ary[mid].val < x) then
      min = mid
    else
      max = mid
    end
  end
  return max
end
local function upper_bound(ary, x, num)
  if num == 0 then return 1 end
  if(x < ary[1].val) then return 1 end
  if(ary[num].val <= x) then return num + 1 end
  local min, max = 1, num
  while(1 < max - min) do
    local mid = mfl((min + max) / 2)
    if(ary[mid].val <= x) then
      min = mid
    else
      max = mid
    end
  end
  return max
end
local n = io.read("*n", "*l")
local t = {}
local str = io.read()
for v in str:gmatch("%d+") do
  local tmp = {}
  tmp.val = tonumber(v)
  tmp.used = false
  table.insert(t, tmp)
end
table.sort(t, function(x, y) return x.val < y.val end)
local cnt = 0
for i = n, 2, -1 do
  if not t[i].used then
    t[i].used = true
    local v = t[i].val
    local mul = 2
    while mul <= v do
      mul = mul * 2
    end
    v = mul - v
    local lb, ub = lower_bound(t, v, i - 1), upper_bound(t, v, i - 1)
    for j = ub - 1, lb, -1 do
      if not t[j].used then
        cnt = cnt + 1
        t[j].used = true
        break
      end
    end
  end
end
print(cnt)
