local n = io.read("*n")
local a = {}
local mean = 0
for i = 1, n do
  a[i] = io.read("*n")
  mean = mean + a[i]
end
mean = mean / n

local allsame = true
for i = 2, n do
  if a[i] ~= a[1] then allsame = false break end
end
if allsame then print("50") os.exit() end

local sq = 0
for i = 1, n do
  sq = sq + (a[i] - mean) * (a[i] - mean)
end
local rmsq = math.sqrt(sq / n)

local m = io.read("*n")
local ret = 50 + 10 * (a[m] - mean) / rmsq
if 0 <= ret then
  print(math.floor(ret))
else
  print(-math.floor(-ret))
end