package main

import "fmt"

func main() {
	var N, v int
	fmt.Scan(&N)
	for i := 0; i < N; i++ {
		var x, y int
		fmt.Scan(&x, &y)
		if v == 0 {
			if y-x > 0 {
				v = y - x
			} else {
				v = -1
			}
		} else if v != y-x {
			v = -1
		}
	}
	fmt.Println(v)
}