package main

import (
	"fmt"
)

var (
	N int
	X []int
	Y []int
)

func resolve() int {
	if X[0] >= Y[0] {
		return -1
	}
	ret := Y[0] - X[0]
	for i := 1; i < N; i++ {
		if Y[i]-X[i] != ret {
			return -1
		}
	}
	return ret
}

func main() {
	fmt.Scanf("%d\n", &N)
	X = make([]int, N)
	Y = make([]int, N)
	for i := 0; i < N; i++ {
		fmt.Scanf("%d %d\n", &X[i], &Y[i])
	}
	fmt.Println(resolve())
}