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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 122060484 569384656  18% /
tmpfs                              12182492       252  12182240   1% /dev/shm
tmpfs                               2436500      1640   2434860   1% /run
tmpfs                                  5120         8      5112   1% /run/lock
/dev/sda2                           1992552    212508   1658804  12% /boot
tmpfs                               2436496       136   2436360   1% /run/user/1000