Zyflair, all right, to double check to see if I understand you correctly, let's use my aforementioned example of trying to flip a heads in 10 coins.
X is 2 and Y is 10, so...
1 - ((2-1)/2) ^ 10
1 - 0.5^10
0.5 to the 10th is .0009765625, so subtract that from 1 and I get 99.90234375%.
...which...makes sense, actually. Thank you!