Peter's question about forgiveness

Matt 18:21 Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him? till seven times?

Matt 18:22a Jesus saith unto him,

Matt 18:22b I say not unto thee, Until seven times: but, Until seventy times seven [1]. Luke 17:4 And if he trespass against thee seven times in a day, and seven times in a day turn again to thee, saying, I repent; thou shalt forgive him.

[1]Genesis 4:24 If Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.