def main():
    N = int(input())
    XY = tuple(tuple(map(int, input().split())) for _ in [0] * N)

    z = XY[0][1] - XY[0][0]
    for x, y in XY:
        if z != y - x or z <= 0:
            print(-1)
            break
    else:
        print(z)
            
main()