Dear, sirs
Could you help to find out the reason of WA ? // thank you

import java.util.*;

public class sumN {

public static void main(String[] args) {
  Scanner in = new Scanner(System.in);
  int k = 1;
  int sum = 0;
  int N = in.nextInt();
  int M = Math.abs(N);
if (M<=10000) {
  int arr[] = new int[M-1];

     for (int i=0; i<(M-1); i++) {
        k++;
        arr[i]=k;
                            }

        for (int i=0; i<(M-1); i++)
            sum += arr[i];
            System.out.print(sum);
}
}
}

also I tried

import java.util.*;

public class sumM {

public static void main(String[] args) {
  Scanner in = new Scanner(System.in);
  int sum = 0;
  int N = in.nextInt();
  int M = Math.abs(N);
if (M<=10000) {

     for (int i=1; i<=(M); i++)
        sum += i;
        System.out.print(sum-1);

}
}
}

Edited by author 02.10.2013 10:19

solved

import java.util.*;

public class sumM {

public static void main(String[] args) {
  Scanner in = new Scanner(System.in);
  int sum = 0;
  int N = in.nextInt();
  if ((N>0) & (Math.abs(N)<=10000)) {

     for (int i=1; i<=N; i++) sum += i;

    }
     else if ((N<=0) & (Math.abs(N)<=10000))
    {

     for (int i=N; i<=1; i++) sum += i;

                    }
        System.out.print(sum);
}
}

this solution doesn't give the right answer