local mfl, mce = math.floor, math.ceil
local mmi, mma = math.min, math.max
local n = io.read("*n")
local a, b = {}, {}
a[1], b[1] = 0, 0
for i = 1, n do
  local v = io.read("*n")
  if v == 0 then
   a[#a] = a[#a] + 1
  elseif v == 1 then
    b[#b] = b[#b] + 1
  else
    table.insert(a, 0)
    table.insert(b, 0)
  end
end
n = #a
local dp1, dp2 = {}, {}
local minval = 0
local maxval = 0
dp1[1] = 0
local inf = 1000000007
for i = 1, n do
  local src = i % 2 == 1 and dp1 or dp2
  local dst = i % 2 == 1 and dp2 or dp1
  local nxtmin = minval - a[i]
  local nxtmax = maxval + b[i]
  for j = nxtmin, nxtmax do
    dst[j - nxtmin + 1] = inf
  end
  for j = minval, maxval do
   local v = src[j - minval + 1]
   local d1 = j - a[i]
   dst[d1 - nxtmin + 1] = mmi(dst[d1 - nxtmin + 1], v + a[i])
   local d2 = j + b[i]
   dst[d2 - nxtmin + 1] = mmi(dst[d2 - nxtmin + 1], v + b[i])
  end
  minval, maxval = nxtmin, nxtmax
end
local tbl = n % 2 == 1 and dp2 or dp1
local ret = tbl[0 - minval + 1]
print(ret < inf and mfl(ret / 2) or -1)