n = int(input())
x1, y1 = map(int, input().split())
z = max(y1 - x1, -1)
n -= 1
if n <= 0 or z == -1:
    print(z)
else:
    for _ in range(n):
        x, y = map(int, input().split())
        tz = max(y - x, -1)
        if tz != z:
            z = -1
            break
    print(z)