sum = n/2(2a+(n-1)d)

here sum = given number ...

n = number of terms/ here # of stairs

a = first term = 1

d= difference = 1

make an equation and solve for n^2 + n -2*n = 0

root = (-b + sqrt(4ac))/2...

constant for us a = 1, b =1 so just solve it convert it to int

`return int((math.sqrt(1+8*n)-1)/2)`