package main

import "fmt"

func main() {
	var N, X, Y, rlt int
	fmt.Scan(&N)
	fmt.Scan(&X, &Y)
	rlt = Y - X
	if rlt <= 0 {
		fmt.Println("-1")
		return
	}
	for i := 0; i < N-1; i++ {
		fmt.Scan(&X, &Y)
		if rlt != Y-X {
			fmt.Println("-1")
			return
		}
	}
	fmt.Println(rlt)
}