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