local n = io.read("*n")
local a = {}
for i = 1, n do
  a[i] = io.read("*n") - i
end
if n == 1 then
  print(0)
else
  table.sort(a)
  local hn = n // 2
  local pos = a[hn]
  local ret = 0
  for i = 1, hn do
    ret = ret + pos - a[i]
  end
  for i = hn + 1, n do
    ret = ret + a[i] - pos
  end
  print(ret)
end
