Summation formula

[Jmoz092]

Hi, I need help creating a worksheet formula. I have a task with tiered goals (0-80). One may be evaluated at any point on this tier; employee A may start at tier 1, employee B may start at tier 10 at evaluation times. Going from tier 0 to tier 1 requires satisfactory completion of 1 task. Going from tier 1 to tier 2 requires satisfactory completion of 1 tasks. Going from tier 2 to tier 3 requires satisfactory completion of 1 tasks so forth and so on. So to get to tier 3, the employee would have needed to complete 6 tasks, to go from tier 0 to tier 20, the employee would need to have completed 210 tasks.

I need a way to figure out how many tasks the employee needs to complete given the current tier they're on, and their goal. The scale goes to 80, but it is not always obtainable. [TABLE="class: grid, width: 500"]
<tbody>[TR]
[TD]Current Tier[/TD]
[TD]Tier Goal[/TD]
[TD]# of completed tasks needed[/TD]
[/TR]
[TR]
[TD]0[/TD]
[TD]1[/TD]
[TD]1[/TD]
[/TR]
</tbody>[/TABLE]

A formula to calculate C2 based on variable input into A2 and B2 is what I'm after. I've tried adding helper columns to use Gaussian summation, but I'm not getting where I need to be. Any help would be appreciated

 

[RedWolf3x]

The Formula

=(Tier Goal*(Tier Goal + 1)/2)-(Current Tier*(Current Tier + 1)/2)

Should do it I think

 

[Jmoz092]

works beautifully, thank you very much