Name       : F_BACKSTAB_PEN(bs, knife, wp, str)

Defined in : <formulas.h>

Description: Calculate the penetration value of the backstab attempt.

Arguments  : (int)bs    - the backstab skill of the attacker.
             (int)knife - the knife skill of the attacker.
             (int)wp    - the penetration value of the knife.
             (int)str   - the strength of the attacker.

Returns    : (int) the penetration value.