func main()
	var n: int :: cui@inputInt()
	var sum: int :: 0
	var max: int :: 0
	var mod2: int
	for i(1, n)
		var a: int :: cui@inputInt()
		var b: int :: cui@inputInt()
		if(i = 1)
			do mod2 :: a % 2
		elif(a % 2 <> mod2)
			do cui@print("-1\n")
			ret
		end if
		do max :: [a + 4 * b, max].max()
		do sum :+ a + 4 * b
	end for
	var ans: int :: (max * n - sum) / 2
	do cui@print("\{ans}\n")
end func