I hope it's ok to ask this here.

I'm working on an role playing game and I need help with a combat formula.

What formula would make all 3 of these statements true?

1) If attacker has Weapon Skill of 100 (maximum) and defender has Parry skill of 0 (minimum), then attacker has 100% chance of hitting defender.

2) If attacker has Weapon Skill of 0 (minimum) and defender has Parry skill of 100 (maximum), then attacker has 0% chance of hitting defender.

3) If Weapon Skill = Parry Skill, whether both 0 or both 100 (or anything in between) attacker has 50% chance of hitting defender.

So I need to be able to figure out the chance to hit for 0-100 vs 0-100

eg. attacker has a Weapon skill of 73 and the defender Parry skill of 29