#include <bits/stdc++.h>
using namespace std;
const int N = 505;
const int mod = 1000000007;
int n, K, mi[N * N], dp[N][N];
void pp(int &x, int y) {
  x += y;
  if (x >= mod) x -= mod;
}
int main() {
  scanf("%d%d", &n, &K);
  mi[0] = 1;
  for (int i = 1; i < N * N; i++) mi[i] = mi[i - 1] * 2 % mod;
  dp[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    dp[i][0] = 1;
    for (int j = 1; j <= i; j++) {
      for (int k = j - 1; k < i; k++) {
        pp(dp[i][j], 1ll * dp[k][j - 1] * (mi[i - k] - 1 + mod) % mod *
                         mi[(i - k) * k] % mod);
      }
    }
  }
  printf("%d\n", dp[n][K]);
  return 0;
}
