Problem: Let's assume that set S consists of m distinct intervals [l1, r1], [l2, r2], ..., [lm, rm] (1 ≤ li ≤ ri ≤ n; li, ri are integers).

Let's assume that f(S) is the maximum number of intervals that you can choose from the set S, such that every two of them do not intersect. We assume that two intervals, [l1, r1] and [l2, r2], intersect if there is an integer x, which meets two inequalities: l1 ≤ x ≤ r1 and l2 ≤ x ≤ r2.

Sereja wonders, how many sets S are there, such that f(S) = k? Count this number modulo 1000000007 (109 + 7).

Input

The first line contains integers n, k (1 ≤ n ≤ 500; 0 ≤ k ≤ 500).

Output

In a single line, print the answer to the problem modulo 1000000007 (109 + 7).

Examples

Input

3 1


Output

23


Input

3 2


Output

32


Input

2 0


Output

1


Input

2 2


Output

2

Solution (in JAVA):

Solution:
import java.io.OutputStreamWriter;
import java.io.BufferedWriter;
import java.util.Comparator;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Random;
import java.io.Writer;
import java.util.List;
import java.io.IOException;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.ArrayList;
import java.util.NoSuchElementException;
import java.math.BigInteger;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author Egor Kulikov (egor@egork.net)
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		InputReader in = new InputReader(inputStream);
		OutputWriter out = new OutputWriter(outputStream);
		TaskE solver = new TaskE();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskE {
	private static final long MOD = (long) (1e9 + 7);
	long[][] answer;
	long[] p = IntegerUtils.generatePowers(2, 500 * 501 / 2 + 2, MOD);

    public void solve(int testNumber, InputReader in, OutputWriter out) {
		int size = in.readInt();
		int count = in.readInt();
		if (size < count) {
			out.printLine(0);
			return;
		}
		answer = new long[size + 1][count + 1];
		ArrayUtils.fill(answer, -1);
		out.printLine(calculate(size, count));
    }

	private long calculate(int size, int count) {
		if (count == 0)
			return 1;
		if (answer[size][count] != -1)
			return answer[size][count];
		answer[size][count] = 0;
		for (int i = size - count + 1; i > 0; i--) {
			int nSize = size - i;
			int minIntersect = nSize * i;
			int maxIntersect = nSize * i + i;
			answer[size][count] += (p[maxIntersect] - p[minIntersect]) * calculate(nSize, count - 1) % MOD;
		}
		answer[size][count] %= MOD;
		if (answer[size][count] < 0)
			answer[size][count] += MOD;
		return answer[size][count];
	}
}

class IntegerUtils {

	public static long[] generatePowers(long base, int count, long mod) {
		long[] result = new long[count];
		if (count != 0)
			result[0] = 1 % mod;
		for (int i = 1; i < count; i++)
			result[i] = result[i - 1] * base % mod;
		return result;
	}

}

class InputReader {

	private InputStream stream;
	private byte[] buf = new byte[1024];
	private int curChar;
	private int numChars;
	private SpaceCharFilter filter;

	public InputReader(InputStream stream) {
		this.stream = stream;
	}

	public int read() {
		if (numChars == -1)
			throw new InputMismatchException();
		if (curChar >= numChars) {
			curChar = 0;
			try {
				numChars = stream.read(buf);
			} catch (IOException e) {
				throw new InputMismatchException();
			}
			if (numChars <= 0)
				return -1;
		}
		return buf[curChar++];
	}

	public int readInt() {
		int c = read();
		while (isSpaceChar(c))
			c = read();
		int sgn = 1;
		if (c == '-') {
			sgn = -1;
			c = read();
		}
		int res = 0;
		do {
			if (c < '0' || c > '9')
				throw new InputMismatchException();
			res *= 10;
			res += c - '0';
			c = read();
		} while (!isSpaceChar(c));
		return res * sgn;
	}

	public boolean isSpaceChar(int c) {
		if (filter != null)
			return filter.isSpaceChar(c);
		return isWhitespace(c);
	}

	public static boolean isWhitespace(int c) {
		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
	}

	public interface SpaceCharFilter {
		public boolean isSpaceChar(int ch);
	}
}

class OutputWriter {
	private final PrintWriter writer;

	public OutputWriter(OutputStream outputStream) {
		writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
	}

	public OutputWriter(Writer writer) {
		this.writer = new PrintWriter(writer);
	}

	public void close() {
		writer.close();
	}

	public void printLine(long i) {
		writer.println(i);
	}

	public void printLine(int i) {
		writer.println(i);
	}
}

class ArrayUtils {

	public static void fill(long[][] array, long value) {
		for (long[] row : array)
			Arrays.fill(row, value);
	}

}