Run Time Error in program

Hello all
I am trying to solve a problem on
http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=12&page=show_problem&problem=964
but i am consistently getting runtime error. I asked about this on
forum of respective site but no luck so i am asking here.
Thank you

/*
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//package uvaproblem_10023;
import java.io.*;
import java.math.BigInteger;

/**
 *
 * @author user
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
       try
       {
         BufferedReader stdin= new BufferedReader(new
InputStreamReader(System.in));
         String s_1=stdin.readLine();
        // stdin.readLine();//for blank line
         int n=Integer.parseInt(s_1);
         boolean f=false;
         for(int i=0;i<n;i++)
         {
            if(f)System.out.println();
            f=true;
            String s_2=stdin.readLine();
            BigInteger x=new BigInteger(s_2);
            BigInteger lo=BigInteger.ZERO,hi=x,mid;
            while(lo.compareTo(hi)<0)
            {
               mid=(lo.add(hi)).divide(new BigInteger("2"));
               BigInteger res=mid.pow(2);
               if(res.compareTo(x)<0) lo=mid.add(new BigInteger("1"));
               else hi=mid;

            }
            mid=(lo.add(hi)).divide(new BigInteger("2"));
            //System.out.println(lo+" "+mid+" "+hi+" "+mid.pow(2));
            System.out.println(mid);
         }

       }catch (NumberFormatException e){}
       catch(IOException e){}
    }

}