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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 181122972 510322168  27% /
tmpfs                              12182504       168  12182336   1% /dev/shm
tmpfs                               2436504      1724   2434780   1% /run
tmpfs                                  5120         8      5112   1% /run/lock
/dev/sda2                           1992552    313524   1557788  17% /boot
tmpfs                               2436500       112   2436388   1% /run/user/131
tmpfs                               2436500        12   2436488   1% /run/user/1000