n = int(input())

tmp_list = []
for i in range(n):
    x, y = map(int, input().split())

    tmp = y - x
    if tmp > 0:
        tmp_list.append(tmp)
    else:
        print(-1)
        break
else:
    if tmp_list.count(tmp_list[0]) == n:
        print(tmp_list[0])
    else:
        print(-1)