Why ssdump.dat file obtained from gmx do_dssp has one missing residue?

GROMACS version: 2018.6
GROMACS modification: No
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Dear Gromacs Users:

Following MD simulation, I have calculated secondary structure using gmx do_dssp. following is the command I used:
gmx do_dssp -f ptn_1000ns_dt100_rep0_rgg.xtc -s 2md0.tpr -o ss.xpm -ssdump ssdump.dat -dt 1000

This ran fine without any error. The protein has 53 residues in total. However, when I checked the ssdump.dat (which has the secondary structure information for each residue), it has 52 residues instead of 53.

I would really appreciate it if someone tells me why one residue is missing in the secondary structure information file and from which end of the residue it is missing.
Thank you all in advance.

Following is an excerpt from the above-mentioned file.