package main

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

func main() {
	sc := NewScanner(os.Stdin)
	N := sc.NextInt()

	zz := -1
	for i := 0; i < N; i++ {
		X := sc.NextInt()
		Y := sc.NextInt()
		z := Y - X
		if z <= 0 {
			fmt.Println(-1)
			return
		}
		if i == 0 {
			zz = z
			continue
		}
		if zz != z {
			fmt.Println(-1)
			return
		}
	}
	fmt.Println(zz)
}

type Scanner struct {
	*bufio.Scanner
}

func NewScanner(r io.Reader) *Scanner {
	return &Scanner{
		bufio.NewScanner(r),
	}
}

func (s *Scanner) Next() string {
	sc := s.Scanner
	sc.Split(bufio.ScanWords)
	return nextToken(sc)
}

func nextToken(sc *bufio.Scanner) string {
	sc.Scan()
	if sc.Err() != nil {
		panic(sc.Err())
	}
	return sc.Text() // not handle EOF
}

func (s *Scanner) NextInt() int {
	i, e := strconv.Atoi(s.Next())
	if e != nil {
		panic(e)
	}
	return i
}