Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

 

In this case, you should make an educated guess and then make up the fasts you missed accordingly.  You will not be punished if you make mistakes.

 

We read in the Qur’an:

“Allah does not burden any human being with more than he is well able to bear: in his favor shall be whatever good he does, and against him whatever evil he does. O our Lord! Take us not to task if we forget or unwittingly do wrong! “O our Lord! Lay not upon us a burden such as Thou didst lay upon those who lived before us! O our Lord! Make us not bear burdens which we have no strength to bear! “And efface Thou our sins, and grant us forgiveness, and bestow Thy mercy upon us! Thou art our Lord Supreme: succor us, then, against people who deny the truth!” (Al-Baqarah 2: 286)

 

And the Prophet (peace be upon him) said, “Allah has excused my community of their unintentional mistakes through ignorance and forgetfulness.”

 

Almighty Allah knows best.