n = int(input())

x_y_lst = [ list(map(int, input().split(' '))) for i in range(n) ]

pre_diff = -1
for i, x_y in enumerate(x_y_lst):
    x = x_y[0]
    y = x_y[1]

    if x >= y:
        pre_diff = -1
        break
    if i != 0:
        if y - x != pre_diff:
            pre_diff = -1
            break
    pre_diff = y-x

print(pre_diff)