class Problem0185:
	def solve(this):
		n = int(input())
		
		c = True
		res = set()
		for i in range(n):
			x, y = map(int, input().split())
			if x >= y:
				c = False
			res.add(y - x)
		if c and len(res) == 1:
			t = list(res)
			print(t[0])
		else:
			print(-1)
			
if __name__ == "__main__":
	problem = Problem0185()
	problem.solve()