// No.185 和風
package main

import "fmt"

func main() {
	var n int
	fmt.Scan(&n)

	type Data struct {
		X int
		Y int
	}
	list := make([]Data, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&list[i].X, &list[i].Y)
	}

	ans := max(list[0].Y-list[0].X, 0)
	for _, v := range list {
		if ans == 0 || ans+v.X != v.Y {
			ans = -1
			break
		}
	}
	fmt.Println(ans)
}