Solo Leveling bash downloader

SoloLevelingDownload.sh

#!/bin/bash

echo "Downloading Solo Leveling..."

start=$(date -d "2018-02-05" '+%s')
today=$(date -d $(date +%Y-%m-%d) '+%s')
diff=$(( ($today - $start) / (60*60*24) ))
last=$(((diff / 7) + (diff % 7 > 0)))

for (( i=0; i<=$last; i++ ))
do
    echo "Downloading Chapter: $i"
    # Download the chapter
    if [ -f "Chapter$i.pdf" ]; then
        echo "Chapter$i.pdf already exists, moving on"
        continue;
    fi
    wget -c https://jaiminisbox.com/reader/download/solo-leveling/en/0/$i/ -O Chapter$i.zip
    download=$?
    if [ $download -eq 0 ]; then
        # Unzip the downloaded chapter
        unzip Chapter$i.zip -d Chapter$i
        # Convert to PDF
        convert Chapter$i/* Chapter$i.pdf
        # Clean up
        rm -rf Chapter$i/ Chapter$i.zip
        # blaber to user
        echo "Chapter$i downloaded and converted to PDF."
    else
        echo "Download failed, stopping at chapter: $i";
        break;
    fi

done