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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 95623712 595821428  14% /
tmpfs                              12182508      252  12182256   1% /dev/shm
tmpfs                               2436504     4840   2431664   1% /run
tmpfs                                  5120        8      5112   1% /run/lock
tmpfs                               2436500      112   2436388   1% /run/user/131
tmpfs                               2436500       96   2436404   1% /run/user/1000
/dev/sda2                           1992552   304460   1566852  17% /boot