package main

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

func main() {
	var N, X, Y, rlt int
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Scan()
	N, _ = strconv.Atoi(sc.Text())
	sc.Scan()
	X, _ = strconv.Atoi(sc.Text())
	sc.Scan()
	Y, _ = strconv.Atoi(sc.Text())
	rlt = Y - X
	if rlt <= 0 {
		fmt.Println("-1")
		return
	}
	for i := 0; i < N-1; i++ {
		sc.Scan()
		X, _ = strconv.Atoi(sc.Text())
		sc.Scan()
		Y, _ = strconv.Atoi(sc.Text())
		if rlt != Y-X {
			fmt.Println("-1")
			return
		}
	}
	fmt.Println(rlt)
}