请帮忙将文件推送到gitlab，当尝试使用coomand git push -u origin master时我遇到下一个问题
$ git push -u origin master
To https://gitlab.com/bachelor2019/pi-52/yarmolenko-denis.git
! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to 'https://gitlab.com/bachelor2019/pi-52/yarmolenko-denis.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.