15.已知数列{an}\{a_{n}\}{an}的前nnn项和为SnS_{n}Sn,且满足Sm+Sn=Sm+nS_{m}+S_{n}=S_{m+n}Sm+Sn=Sm+n,若a1=2a_{1}=2a1=2,则a20=(a_{20}=(a20=( )))