n = int(input())
ans_set = set()
for _ in range(n):
    x, y = map(int, input().split())
    res = y - x
    if res <= 0:
        print(-1)
        exit()
    ans_set.add(res)
if len(ans_set) == 1:
    for num in ans_set:
        print(num)
else:
    print(-1)