local n = io.read("*n")
local tot = 0
local a = {}
for i = 1, n do
  local tmp = io.read("*n")
  tot, a[i] = tot + tmp, tmp
end
local minmvcnt = 10000
for i = 1, 100 do
  if(tot < i * i) then break end
  local mvcnt = 0
  -- check only MINUS count
  for j = 1, n do
    local tgt = 0
    if(j <= i) then tgt = j
    elseif(j < 2 * i) then tgt = 2 * i - j
    end
    mvcnt = mvcnt + math.max(0, a[j] - tgt)
  end
  minmvcnt = math.min(minmvcnt, mvcnt)
end
print(minmvcnt)