for i in range(N):
    x, y = input().split()
    x, y = int(x), int(y)
    difference = y - x
    if i == 0:
        saved_difference = difference
    else:
        if saved_difference != difference:
            difference = -1
            break
        else:
            pass


if difference <= 0:
    print(difference)
else:
    print(difference)