local n = io.read("*n")
local a, b = io.read("*n", "*n")
local max = a + b * 4
local mod2 = a % 2
local muri = false
local t = {}
t[1] = max
for i = 2, n do
  a, b = io.read("*n", "*n")
  local v = a + b * 4
  t[i] = v
  max = math.max(max, v)
  if(mod2 ~= a % 2) then
    muri = true
    print(-1)
    break
  end
end
if(not muri) then
  local ret = 0
  for i = 1, n do
    ret = ret + (max - t[i]) / 2
  end
  print(ret)
end