If pid is negative but not -1, the signal is sent to all processes whose process group ID is equal to the absolute value of pid. The negative pid is specified in this way:
kill -s KILL -- -nn
where nn is the process group ID and may have a range of 2 to 7 digits (nn to nnnnnnn).
kill -s KILL -- -9812753
The format must include the – – – before the nn in order to specify the process group ID