Problem:
Score : 900 points

Problem Statement

Given are a permutation p_1, p_2, \dots, p_N of (1, 2, ..., N) and an integer K. Maroon performs the following operation for i = 1, 2, \dots, N - K + 1 in this order:

Shuffle p_i, p_{i + 1}, \dots, p_{i + K - 1} uniformly randomly.

Find the expected value of the inversion number of the sequence after all the operations are performed, and print it modulo 998244353.

More specifically, from the constraints of this problem, it can be proved that the expected value is always a rational number, which can be represented as an irreducible fraction \frac{P}{Q}, and that the integer R that satisfies R \times Q \equiv P \pmod{998244353}, 0 \leq R < 998244353 is uniquely determined. Print this R.

Here, the inversion number of a sequence a_1, a_2, \dots, a_N is defined to be the number of ordered pairs (i, j) that satisfy i < j, a_i > a_j.

Constraints

2 \leq N \leq 200,000

2 \leq K \leq N

(p_1, p_2, \dots, p_N) is a permutation of (1, 2, \dots, N).

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N K
p_1 p_2 ... p_N

Output

Print the expected value modulo 998244353.

Sample Input 1

3 2
1 2 3

Sample Output 1

1

The final sequence is one of (1, 2, 3), (2, 1, 3), (1, 3, 2), (2, 3, 1), each with probability \frac{1}{4}.
Their inversion numbers are 0, 1, 1, 2 respectively, so the expected value is 1.

Sample Input 2

10 3
1 8 4 9 2 3 7 10 5 6

Sample Output 2

164091855

Solution in Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.io.Closeable;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 29);
        thread.start();
        thread.join();
    }

    static class TaskAdapter implements Runnable {
        @Override
        public void run() {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            FastInput in = new FastInput(inputStream);
            FastOutput out = new FastOutput(outputStream);
            EShuffleWindow solver = new EShuffleWindow();
            solver.solve(1, in, out);
            out.close();
        }
    }

    static class EShuffleWindow {
        private static long mod = 998244353;
        private static Modular modular = new Modular(mod);
        private static Power power = new Power(modular);

        public void solve(int testNumber, FastInput in, FastOutput out) {
            int n = in.readInt();
            int k = in.readInt();
            int[] p = new int[n];
            in.populate(p);
            Segment segment = new Segment(1, n);
            int die = power.inverse(k);
            int alive = modular.subtract(1, die);
            int half = power.inverse(2);
            long exp = 0;
            long inverseCnt = 0;
            for (int i = 0; i < n; i++) {
                if (i >= k) {
                    segment.update(1, n, 1, n, alive);
                }
                long pre = segment.query(1, p[i], 1, n);
                long post = segment.query(p[i], n, 1, n);
                exp += half * pre % mod;
                exp -= half * post % mod;

                int postSize = segment.querySize(p[i], n, 1, n);
                inverseCnt += postSize;

                segment.active(p[i], p[i], 1, n);
            }
            long ans = exp + inverseCnt;
            ans = modular.valueOf(ans);
            out.println(ans);
        }

    }

    static class FastInput {
        private final InputStream is;
        private byte[] buf = new byte[1 << 13];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastInput(InputStream is) {
            this.is = is;
        }

        public void populate(int[] data) {
            for (int i = 0; i < data.length; i++) {
                data[i] = readInt();
            }
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    bufLen = -1;
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

    }

    static class FastOutput implements AutoCloseable, Closeable, Appendable {
        private StringBuilder cache = new StringBuilder(10 << 20);
        private final Writer os;

        public FastOutput append(CharSequence csq) {
            cache.append(csq);
            return this;
        }

        public FastOutput append(CharSequence csq, int start, int end) {
            cache.append(csq, start, end);
            return this;
        }

        public FastOutput(Writer os) {
            this.os = os;
        }

        public FastOutput(OutputStream os) {
            this(new OutputStreamWriter(os));
        }

        public FastOutput append(char c) {
            cache.append(c);
            return this;
        }

        public FastOutput append(long c) {
            cache.append(c);
            return this;
        }

        public FastOutput println(long c) {
            return append(c).println();
        }

        public FastOutput println() {
            cache.append(System.lineSeparator());
            return this;
        }

        public FastOutput flush() {
            try {
                os.append(cache);
                os.flush();
                cache.setLength(0);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            return this;
        }

        public void close() {
            flush();
            try {
                os.close();
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        public String toString() {
            return cache.toString();
        }

    }

    static interface InverseNumber {
    }

    static class Segment implements Cloneable {
        private Segment left;
        private Segment right;
        private static long mod = 998244353;
        private long sum = 0;
        private long mul = 1;
        private int size = 0;

        private void active() {
            size = 1;
            sum = 1;
        }

        private void modify(long x) {
            sum = sum * x % mod;
            mul = mul * x % mod;
        }

        public void pushUp() {
            sum = (left.sum + right.sum) % mod;
            size = left.size + right.size;
        }

        public void pushDown() {
            if (mul != 1) {
                left.modify(mul);
                right.modify(mul);
                mul = 1;
            }
        }

        public Segment(int l, int r) {
            if (l < r) {
                int m = DigitUtils.floorAverage(l, r);
                left = new Segment(l, m);
                right = new Segment(m + 1, r);
                pushUp();
            } else {

            }
        }

        private boolean covered(int ll, int rr, int l, int r) {
            return ll <= l && rr >= r;
        }

        private boolean noIntersection(int ll, int rr, int l, int r) {
            return ll > r || rr < l;
        }

        public void update(int ll, int rr, int l, int r, long x) {
            if (noIntersection(ll, rr, l, r)) {
                return;
            }
            if (covered(ll, rr, l, r)) {
                modify(x);
                return;
            }
            pushDown();
            int m = DigitUtils.floorAverage(l, r);
            left.update(ll, rr, l, m, x);
            right.update(ll, rr, m + 1, r, x);
            pushUp();
        }

        public void active(int ll, int rr, int l, int r) {
            if (noIntersection(ll, rr, l, r)) {
                return;
            }
            if (covered(ll, rr, l, r)) {
                active();
                return;
            }
            pushDown();
            int m = DigitUtils.floorAverage(l, r);
            left.active(ll, rr, l, m);
            right.active(ll, rr, m + 1, r);
            pushUp();
        }

        public long query(int ll, int rr, int l, int r) {
            if (noIntersection(ll, rr, l, r)) {
                return 0;
            }
            if (covered(ll, rr, l, r)) {
                return sum;
            }
            pushDown();
            int m = DigitUtils.floorAverage(l, r);
            return (left.query(ll, rr, l, m) +
                    right.query(ll, rr, m + 1, r)) % mod;
        }

        public int querySize(int ll, int rr, int l, int r) {
            if (noIntersection(ll, rr, l, r)) {
                return 0;
            }
            if (covered(ll, rr, l, r)) {
                return size;
            }
            pushDown();
            int m = DigitUtils.floorAverage(l, r);
            return left.querySize(ll, rr, l, m) +
                    right.querySize(ll, rr, m + 1, r);
        }

        private Segment deepClone() {
            Segment seg = clone();
            if (seg.left != null) {
                seg.left = seg.left.deepClone();
            }
            if (seg.right != null) {
                seg.right = seg.right.deepClone();
            }
            return seg;
        }

        protected Segment clone() {
            try {
                return (Segment) super.clone();
            } catch (CloneNotSupportedException e) {
                throw new RuntimeException(e);
            }
        }

        private void toString(StringBuilder builder) {
            if (left == null && right == null) {
                builder.append("val").append(",");
                return;
            }
            pushDown();
            left.toString(builder);
            right.toString(builder);
        }

        public String toString() {
            StringBuilder builder = new StringBuilder();
            deepClone().toString(builder);
            if (builder.length() > 0) {
                builder.setLength(builder.length() - 1);
            }
            return builder.toString();
        }

    }

    static class Power implements InverseNumber {
        static IntExtGCDObject extGCD = new IntExtGCDObject();
        final Modular modular;
        int modVal;

        public Power(Modular modular) {
            this.modular = modular;
            this.modVal = modular.getMod();
        }

        public int inverse(int x) {
            int ans = inverseExtGCD(x);
//        if(modular.mul(ans, x) != 1){
//            throw new IllegalStateException();
//        }
            return ans;
        }

        public int inverseExtGCD(int x) {
            if (extGCD.extgcd(x, modVal) != 1) {
                throw new IllegalArgumentException();
            }
            return modular.valueOf(extGCD.getX());
        }

    }

    static class Modular {
        int m;

        public int getMod() {
            return m;
        }

        public Modular(int m) {
            this.m = m;
        }

        public Modular(long m) {
            this.m = (int) m;
            if (this.m != m) {
                throw new IllegalArgumentException();
            }
        }

        public Modular(double m) {
            this.m = (int) m;
            if (this.m != m) {
                throw new IllegalArgumentException();
            }
        }

        public int valueOf(int x) {
            x %= m;
            if (x < 0) {
                x += m;
            }
            return x;
        }

        public int valueOf(long x) {
            x %= m;
            if (x < 0) {
                x += m;
            }
            return (int) x;
        }

        public int subtract(int x, int y) {
            return valueOf(x - y);
        }

        public String toString() {
            return "mod " + m;
        }

    }

    static class IntExtGCDObject {
        private int[] xy = new int[2];

        public int extgcd(int a, int b) {
            return ExtGCD.extGCD(a, b, xy);
        }

        public int getX() {
            return xy[0];
        }

    }

    static class ExtGCD {
        public static int extGCD(int a, int b, int[] xy) {
            if (a >= b) {
                return extGCD0(a, b, xy);
            }
            int ans = extGCD0(b, a, xy);
            SequenceUtils.swap(xy, 0, 1);
            return ans;
        }

        private static int extGCD0(int a, int b, int[] xy) {
            if (b == 0) {
                xy[0] = 1;
                xy[1] = 0;
                return a;
            }
            int ans = extGCD0(b, a % b, xy);
            int x = xy[0];
            int y = xy[1];
            xy[0] = y;
            xy[1] = x - a / b * y;
            return ans;
        }

    }

    static class DigitUtils {
        private DigitUtils() {
        }

        public static int floorAverage(int x, int y) {
            return (x & y) + ((x ^ y) >> 1);
        }

    }

    static class SequenceUtils {
        public static void swap(int[] data, int i, int j) {
            int tmp = data[i];
            data[i] = data[j];
            data[j] = tmp;
        }

    }
}