Combine multiple pdfs into one pdf

gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=chem120_assnt1.pdf -dBATCH ./1638_001.pdf ./1638_002.pdf ./1638_003.pdf ./1638_004.pdf ./1638_005.pdf ./1638_006.pdf ./1638_007.pdf ./1638_008.pdf ./1638_009.pdf ./1638_010.pdf ./1638_011.pdf ./1638_012.pdf ./1638_013.pdf ./1638_014.pdf ./1638_015.pdf ./1638_016.pdf