Which of the following will correctly calculate the free slack in an activity?

$1-$ Late finish of the activity $-$ late start of the next activity

$2-$ Late start of the activity $-$Earl start of the activity

$3-$ late finish of the predecessor $-$ early start of the successor

$4-$ Minimum early start of all the successors $-$ early finish of the activity

$5-$ Either IS $-$ ES or LF $-$ EF