Problem:
Balls and Boxes 4

Balls

Boxes

Any way

At most one ball

At least one ball

Distinguishable

Distinguishable

1

2

3

Indistinguishable

Distinguishable

4

5

6

Distinguishable

Indistinguishable

7

8

9

Indistinguishable

Indistinguishable

10

11

12

Problem

You have $n$ balls and $k$ boxes. You want to put these balls into the boxes.

Find the number of ways to put the balls under the following conditions:

Each ball is not distinguished from the other.

Each box is distinguished from the other.

Each ball can go into only one box and no one remains outside of the boxes.

Each box can contain an arbitrary number of balls (including zero).

Note that you must print this count modulo $10^9+7$.

Input

$n$ $k$

The first line will contain two integers $n$ and $k$.

Output

Print the number of ways modulo $10^9+7$ in a line.

Constraints

$1 \le n \le 1000$

$1 \le k \le 1000$

Sample Input 1

5 3

Sample Output 1

21

Sample Input 2

10 5

Sample Output 2

1001

Sample Input 3

100 100

Sample Output 3

703668401

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Console;
using static System.Convert;
using static System.Math;
//using Debug;
//using static System.Globalization.CultureInfo;
using System.Text;
class Program
{
    static void Main(string[] args)
    {
        Solve();
        //WriteLine(Solve());
    }
    static void Solve()
    {
        var nk = Input.ar;
        var comb = new Modulo.Combination(nk[0] + nk[1]);
        WriteLine(comb.Comb(nk[0] + nk[1] - 1, nk[0]));
    }
}

public class Input
{
    public static string read => ReadLine();
    public  static int[] ar => Array.ConvertAll(read.Split(' '), int.Parse);
    public  static int num => ToInt32(read);
    public static long[] arL => Array.ConvertAll(read.Split(' '), long.Parse);
    public  static long numL => ToInt64(read);
    public static char[][] gred(int h) 
        => Enumerable.Repeat(0, h).Select(v => read.ToCharArray()).ToArray();
    public static int[][] ar2D(int num)
        => Enumerable.Repeat(0, num).Select(v => ar).ToArray();
    public static long[][] arL2D(int num)
        => Enumerable.Repeat(0, num).Select(v => arL).ToArray();
    public static T getValue<T>(string g)
    {
        var t = typeof(T);
        if (t == typeof(int))
            return (T)(object)int.Parse(g);
        if (t == typeof(long))
            return (T)(object)long.Parse(g);
        if (t == typeof(string))
            return (T)(object)g;
        if (t == typeof(char))
            return (T)(object)char.Parse(g);
        if (t == typeof(double))
            return (T)(object)double.Parse(g);
        if (t == typeof(bool))
            return (T)(object)bool.Parse(g);
        return default(T);
    }
    public const long Inf = (long)1e18;
    public const double eps = 1e-6;
    public  const string alfa = "abcdefghijklmnopqrstuvwxyz";
    public  const int MOD = 1000000007;
}
public class Modulo
{
    private static int Multiple(int num1, int num2)
        => (int)(BigMul(num1, num2) % Input.MOD);

    public static int Pow(int m, int n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Pow(Multiple(m, m), n / 2);
        else return Multiple(Pow(Multiple(m, m), n / 2), m);
    }

    public static int Div(int a, int b)
        => Multiple(a, Pow(b, Input.MOD - 2));

    public class Combination
    {
        private int[] _fac;
        public Combination(int num)//階乗をメモ化
        {
            _fac = new int[num + 1];
            _fac[0] = 1;
            for (var i = 1; i <= num; i++)
                _fac[i] = Multiple(_fac[i - 1], i);
        }
        public int Comb(int n, int r)
        {
            if (n < r) return 0;
            if (n == r) return 1;
            var calc = _fac[n];
            calc = Div(calc, _fac[r]);
            calc = Div(calc, _fac[n - r]);
            return calc;
        }
        public int fac(int num)
            => _fac[num];
    }

}