local n = io.read("*n")
local tm = {}
for i = 1, n do
  local a = io.read("*n")
  if not tm[a] then tm[a] = 1
  else tm[a] = tm[a] + 1 end
end
local t = {}
for len, cnt in pairs(tm) do
  table.insert(t, {len, cnt})
end
table.sort(t, function(a, b) return a[1] < b[1] end)
local function solve()
  local maxlen = t[#t][1]
  if maxlen % 2 == 0 then
    if t[1][1] ~= maxlen // 2 or t[1][2] ~= 1 then
      return false
    end
    for i = 2, #t do
      if t[i][2] < 2 then
        return false
      end
      if t[i][1] ~= t[i - 1][1] + 1 then
        return false
      end
    end
    return true
  else
    if t[1][1] ~= (maxlen + 1) // 2 or t[1][2] ~= 2 then
      return false
    end
    for i = 2, #t do
      if t[i][2] < 2 then
        return false
      end
      if 1 < i and t[i][1] ~= t[i - 1][1] + 1 then
        return false
      end
    end
    return true
  end
end
print(solve() and "Possible" or "Impossible")
