11614 - Etruscan Warriors Never Play Chess

Difficulty : easy

Solution Description :

Simple math problem

n*(n+1)/2 = s
n^2+n=2*s
n^2+n+(-2*s)=0

Using quadratic equation ax^2+bx+c=0

del = sqrt(b*b-4*a*c) = sqrt(1-4*1*(-2s)) = sqrt(1+8s)
we know x = (-b(+-) del)/2a

Here i use only + value because del is always positive

so n = (-1+sqrt(1+8s))/2

Input as double s
then print n (where n is convert from double to long)