func main()
	var n: int :: cui@input().toInt(&)
	var ans: int :: -1
	for(1, n)
		var x: int :: cui@inputInt()
		var y: int :: cui@inputInt()
		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