public class Sequence {

        public static int what( int[] a, int num )
        {
                boolean[] b = new boolean[ a.length ];

                for ( int i = 0; i != a.length; i++ )
                        if ( ( a[i] % num == 0 ) && ( a[i] / num <= b.length ) )
                            b[ Math.abs( a[i] / num - 1 ) ] = true;

                int count;

                for (
                        count = 0;
                        count != b.length && b[count] == true;
                        count++
                    );

                return count;
        }

        public static void main( String[] argv )
        {
                int[] a1 = { 500, 5, 1, 2, 7, 6, 4, 9 };

                int[] a2 = { 8,2,5,1,3,7 };

                int[] a3 = { 1, 2, 3 };

                int[] a4 = { 500, 5, 1, -2, 7, -6, -4, 9 };

                System.out.println( what( a1, 2 ) );    // 3: 2,4,6

                System.out.println( what( a1, 3 ) );    // 0:

                System.out.println( what( a2, 1 ) );    // 3: 1,2,3

                System.out.println( what( a2, 2 ) );    // 1: 2

                System.out.println( what( a1, 1 ) );    // 2: 1,2

                System.out.println( what( a3, 1 ) );    // 3: 1,2,3

                System.out.println( what( a4, -2 ) );    // 3: 2,4,6

        }
}