n = int(input())
li = []
judge = True

for i in range(n):
    f, s = (int(i) for i in input().split())
    li.append(s - f)

for i in range(len(li)):
    if not li[i] == li[0] or li[i] < 1:
        judge = False
        break

if judge:
    print(li[0])
else:
    print('-1')