#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    x = (x << 1) + (x << 3) + (c ^ 48);
    c = getchar();
  }
  return x * f;
}
const int N = 510, mod = 1e9 + 7;
int p[N * N], f[N][N];
int main() {
  p[0] = 1;
  for (int i = 1; i <= 500 * 500; i++) p[i] = (long long)p[i - 1] * 2 % mod;
  f[0][0] = 1;
  int n = read(), k = read();
  for (int i = 0; i < n; i++) {
    for (int j = 0; j <= k; j++) {
      for (int l = i + 1; l <= n; l++)
        f[l][j + 1] +=
            (long long)f[i][j] * (p[l - i] - 1) % mod * p[(l - i) * i] % mod,
            f[l][j + 1] %= mod;
    }
  }
  int ans = 0;
  for (int i = 0; i <= n; i++)
    ans += (long long)f[i][k] * p[(n - i) * i] % mod, ans %= mod;
  printf("%d", ans);
}
