First.java

    import java.util.*;
    public class test {
        static int n,m;
        static int uniqueIntegerCount = 0;
        static int answer = 0;
        static int[] cnt = new int[10000001];
        static Deque deque = new ArrayDeque<>();
        
        static void insert(int num)
        {
            deque.addLast(num);
            cnt[num]++;
            
            if( cnt[num] == 1 )
                uniqueIntegerCount++;
        }
        
        static void remove(int index)
        {
            // if Subsequence does not fulled.
            if( index < m ) return;
            
            int num = (int)deque.pollFirst();
            cnt[num] --;
            if( cnt[num] == 0 )
                uniqueIntegerCount--;
        }
        
        static void update()
        {
            if( answer < uniqueIntegerCount )
                answer = uniqueIntegerCount;
        }
        
        public static void main(String[] args) {
            Scanner in = new Scanner(System.in);
            n = in.nextInt();
            m = in.nextInt();

            for (int i = 0; i < n; i++) {
                int num = in.nextInt();
                
                insert(num);
                remove(i);
                update();
            }
            
            System.out.println(answer);
        }
    }