func main()
	var N: int :: cui@input().toInt(&)
	var sum: int :: 0
	var max: int :: 0
	var mod2: int
	for n(1, N)
		var _: [][]char :: cui@input().split(" ")
		var a: int :: _[0].toInt(&)
		var b: int :: _[1].toInt(&)
		if(n = 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