Problem:
Score : 1200 points

Problem Statement

N problems have been chosen by the judges, now it's time to assign scores to them!

Problem i must get an integer score A_i between 1 and N, inclusive.
The problems have already been sorted by difficulty: A_1 \le A_2 \le \ldots \le A_N must hold.
Different problems can have the same score, though.

Being an ICPC fan, you want contestants who solve more problems to be ranked higher.
That's why, for any k (1 \le k \le N-1), you want the sum of scores of any k problems to be strictly less than the sum of scores of any k+1 problems.

How many ways to assign scores do you have? Find this number modulo the given prime M.

Constraints

2 \leq N \leq 5000

9 \times 10^8 < M < 10^9

M is a prime.

All input values are integers.

Input

Input is given from Standard Input in the following format:

N M

Output

Print the number of ways to assign scores to the problems, modulo M.

Sample Input 1

2 998244353

Sample Output 1

3

The possible assignments are (1, 1), (1, 2), (2, 2).

Sample Input 2

3 998244353

Sample Output 2

7

The possible assignments are (1, 1, 1), (1, 2, 2), (1, 3, 3), (2, 2, 2), (2, 2, 3), (2, 3, 3), (3, 3, 3).

Sample Input 3

6 966666661

Sample Output 3

66

Sample Input 4

96 925309799

Sample Output 4

83779

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Text;
using System.Linq;

using E = System.Linq.Enumerable;

internal partial class Solver {
    public void Run() {
        var n = ni();
        var m = ni();
        var k = (n - 1) / 2;
        var coef = new int[n + 2];
        // = [1, x0, x1, ... xn-1]
        for (int i = 1; i < 1 + k; i++) {
            coef[0]--;
            coef[i + 2]++;
        }
        for (int i = n - 1; i > n - 1 - k; i--) {
            coef[0]++;
            coef[i + 2]--;
        }
        //coef.Dump();
        // x0 + x1 + x2 ... x{n-1} <= N-1
        // x0 <= N-1-(x1 + x2 ... x{n-1})
        // inner(coef,{x0,..x{n-1}) < A0
        // inner(coef,{x0,..x{n-1}) <= x0
        // |x0| = N-1-(x1 + x2 ... x{n-1}) - inner(coef,{x0,..x{n-1})
        coef[2]++;
        for (int i = 1; i < coef.Length; i++) {
            coef[i] += coef[i - 1];
        }
        var dp = new int[n + 1];
        dp[0] = 1;
        for (int i = 0; i <= n; i++) {
            if (coef[i] == 0) continue;
            for (int j = 0; j <= n; j++) {
                if (j - coef[i] >= 0) {
                    dp[j] += dp[j - coef[i]];
                }
                dp[j] %= m;
            }
        }
        long ans = 0;
        for (int i = 0; i < n; i++) {
            ans += 1L * dp[i] * (n - i);
            ans %= m;
        }
        cout.WriteLine(ans);
    }
}

// PREWRITEN CODE BEGINS FROM HERE
internal partial class Solver : Scanner {
    public static void Main(string[] args) {
#if LOCAL
        byte[] inputBuffer = new byte[1000000];
        var inputStream = Console.OpenStandardInput(inputBuffer.Length);
        using (var reader = new StreamReader(inputStream, Console.InputEncoding, false, inputBuffer.Length)) {
            Console.SetIn(reader);
            new Solver(Console.In, Console.Out).Run();
        }
#else
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver(Console.In, Console.Out).Run();
        Console.Out.Flush();
#endif
    }

#pragma warning disable IDE0052
    private readonly TextReader cin;
    private readonly TextWriter cout;
#pragma warning restore IDE0052

    public Solver(TextReader reader, TextWriter writer)
        : base(reader) {
        cin = reader;
        cout = writer;
    }
    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer) {
    }

#pragma warning disable IDE1006
#pragma warning disable IDE0051
    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private double[] nd(int n) { return NextDoubleArray(n); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
#pragma warning restore IDE1006
#pragma warning restore IDE0051
}

internal static class LinqPadExtension {
    public static T Dump<T>(this T obj) {
#if LOCAL
        return LINQPad.Extensions.Dump(obj);
#else
        return obj;
#endif
    }
}

public class Scanner {
    private readonly TextReader Reader;
    private readonly Queue<string> TokenQueue = new Queue<string>();
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    public Scanner()
        : this(Console.In) {
    }

    public Scanner(TextReader reader) {
        Reader = reader;
    }

    public int NextInt() { return int.Parse(Next(), ci); }
    public long NextLong() { return long.Parse(Next(), ci); }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size) {
        string[] array = new string[size];
        for (int i = 0; i < size; i++) {
            array[i] = Next();
        }

        return array;
    }
    public int[] NextIntArray(int size) {
        int[] array = new int[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextInt();
        }

        return array;
    }

    public long[] NextLongArray(int size) {
        long[] array = new long[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextLong();
        }

        return array;
    }

    public double[] NextDoubleArray(int size) {
        double[] array = new double[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextDouble();
        }

        return array;
    }

    public string Next() {
        if (TokenQueue.Count == 0) {
            if (!StockTokens()) {
                throw new InvalidOperationException();
            }
        }
        return TokenQueue.Dequeue();
    }

    public bool HasNext() {
        if (TokenQueue.Count > 0) {
            return true;
        }

        return StockTokens();
    }

    private static readonly char[] _separator = new[] { ' ', '\t' };
    private bool StockTokens() {
        while (true) {
            string line = Reader.ReadLine();
            if (line == null) {
                return false;
            }

            string[] tokens = line.Split(_separator, StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0) {
                continue;
            }

            foreach (string token in tokens) {
                TokenQueue.Enqueue(token);
            }

            return true;
        }
    }
}