package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func main() {
	lines, max := input()
	result := count(lines, max)
	fmt.Println(result)
}

func input() ([]int, int) {
	N := nextInt()
	lines := make([]int, N)
	max := 0

	for i := 0; i < N; i++ {
		a := nextInt()
		b := nextInt()
		lines[i] = a + b*4
		if lines[i] > max {
			max = lines[i]
		}
	}
	return lines, max
}

func count(lines []int, max int) int {
	result := 0
	for _, w := range lines {
		diff := max - w
		if diff%2 != 0 {
			return -1
		}
		result += diff / 2
	}
	return result
}

var sc = bufio.NewScanner(os.Stdin)

func next() string {
	sc.Split(bufio.ScanWords)
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	i, e := strconv.Atoi(next())
	if e != nil {
		panic(e)
	}
	return i
}