# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N = int(input())
	value = None
	for i in range(N):
		X, Y = getlist()
		if X >= Y:
			print(-1)
			return
		if value == None:
			value = Y - X
		else:
			if value != Y - X:
				print(-1)
				return

	print(value)

if __name__ == '__main__':
	main()