Lets start from starting:
1st to 6th Slow Start.
1st round--> 1 Segment
6th round-->32 segment
After that for some rounds, the segments are increasing linearly (by 1).
16th round--> 42 segments.
17th round --> 20 segmnets // why?? because of 3 duplicate Acks.
Again for some rounds, additive increase is their in congestion window size.
On 23rd round, Congestion window size is 1 segment only .. why?? timout occured.
And what happens on timeout.
Again slow start is followed with new threshold value which is half of the current window size.
26 was the current window size, so half of it will be 13 segments.