I don't know how to correct my program for more speedly work. Help anybody.
import java.util.*;
import java.math.*;

public class InvertedSqrt
{
    public static void main(String []args)
    {
    Scanner scan = new Scanner(System.in);
    ArrayList<BigInteger> input = new    ArrayList<BigInteger>();
    input.add(scan.nextBigInteger());
    while(scan.hasNextBigDecimal() && input.size() * Integer.SIZE <= 256 * 1024 * 1024)
    {
    input.add(scan.nextBigInteger());
    }
    scan.close();
    int i = input.size() - 1;
        while(i >=0)
        {
        System.out.printf("%.4f",java.lang.Math.sqrt(input.get(i).doubleValue()));
        System.out.print("\n");
        i--;
        }
    }
}

in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
      out = new PrintWriter(new OutputStreamWriter(System.out));

use double, but not BigInteger