num = input()

a, b = raw_input().split(" ")
ans = int(b) - int(a)

flag = False
for i in xrange(num-1):
    a, b = raw_input().split(" ")
    tmp = int(b) - int(a)

    if ans != tmp:
        flag = True
        break

if ans < 0 or flag:
    print -1
else:
    print ans