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The output of your command "$ df -k" is:

Filesystem                        1K-blocks      Used Available Use% Mounted on
/dev/mapper/ubuntu--vg-ubuntu--lv 721890576 227792716 463652424  33% /
tmpfs                              12182440       252  12182188   1% /dev/shm
tmpfs                               2436492      2952   2433540   1% /run
tmpfs                                  5120         8      5112   1% /run/lock
tmpfs                               2436488       152   2436336   1% /run/user/1000
/dev/sda2                           1992552    424768   1446544  23% /boot