java - Collision Detection algorithm -

this algorithm i'm trying solve:

there n circular organisms radii r0,r1,..rn-1 . during each second, 2 organisms radii ri , rj , (chosen randomly , equiprobably) collide form new, bigger organism radius ri+rj. enter image description here

given radii n creatures , integer,k, print single floating-point number denoting expected total area of creatures after k seconds.

input format

the first line contains 2 space-separated integers describing respective values of n , k. second line contains n space-separated integers describing respective values of r0,r1,..rn-1

constraints

1 < n < 10^5

0< k < n-1

1 < ri < 10^4

an answer considered correct if has relative error of @ 10^-9.

output format

print single floating-point number denoting expected sum of creatures' areas after k collisions.

sample input 0

3 1

1 2 3

sample output 0

67.0206432766

sample input 1

3 2

1 2 3

sample output 1

113.0973355292

explanation 1

we have n = 3 organisms radii in {1, 2, 3}. after k = 2 seconds, 2 collisions take

place , we're left 1 circle has radius of 6. print total area taken remaining organism, pi (3.14159265359) * 6^2 = 113.0973355292.

i started writing out code folowing:

import java.io.*; import java.util.*; import java.text.*; import java.math.*; import java.util.regex.*;  public class solution {  public static void main(string[] args) {     scanner in = new scanner(system.in);     int n = in.nextint();     int k = in.nextint();      int[] r = new int[n];     for(int r_i=0; r_i < n; r_i++){         r[r_i] = in.nextint();     }      double sum=0;     double avrage=0;     double pi = 3.1415926535;      for(int i=0;i<n;i++){         int sumj=0;             for(int j=0;j<n;j++){                  if(j!=i)                      sumj=sumj+r[j];                      system.out.println(sumj);             }          sum=sum+pi*math.pow((r[i]),2)+pi*math.pow(sumj,2);    }     avrage=sum/n;     system.out.println(avrage); } } 

the problem i'm not sure should use parameter k(seconds) solve problem. works k=1(after second). should change code work k=2(after 2 seconds) or k number(after x seconds)?