n = int(input())

for i in range(n):
    x, y = map(int, input().split())
    ans = y - x
    if ans <= 0:
        print(-1)
        break
    if i > 0:
        if tmp != ans:
            print(-1)
            break
        else:
            tmp = ans
    else:
        tmp = ans
else:
    print(ans)