#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
long long bit[510 * 510];
long long f[510][510];
int main() {
  bit[0] = 1;
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= 500 * 500; i++) bit[i] = (bit[i - 1] * 2) % mod;
  for (int i = 0; i <= n; i++) f[i][0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m && j <= i; j++) {
      for (int k = 0; k < i; k++) {
        if (k < j - 1) continue;
        f[i][j] = (f[i][j] +
                   (f[k][j - 1] * (bit[i - k] - 1)) % mod * bit[(i - k) * k]) %
                  mod;
      }
    }
  }
  cout << f[n][m] << endl;
  return 0;
}
