func main()
	var N: int :: cui@input().toInt(&)
	var ans: int :: -1
	for(1, N)
		var ss: [][]char :: cui@input().split(" ")
		var x: int :: ss[0].toInt(&)
		var y: int :: ss[1].toInt(&)
		if(ans = -1)
			do ans :: y - x
		end if
		if(ans <= 0 | ans <> y - x)
			do cui@print("-1\n")
			ret
		end if
	end for
	do cui@print("\{ans}\n")
end func