#include <bits/stdc++.h>
using namespace std;
const int inf = (int)1e9;
const int mod = inf + 7;
const double eps = 1e-9;
const double pi = acos(-1.0);
long long n, k, two[600], d[600][600];
long long dp[600][600];
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0);
  cin >> n >> k;
  dp[0][0] = two[0] = 1;
  for (int i = 1; i <= n; ++i) {
    two[i] = (two[i - 1] + two[i - 1]);
    if (two[i] >= mod) two[i] -= mod;
  }
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j <= k; ++j) {
      for (int last = 0; last <= i; ++last) {
        d[j][last] = dp[j][last];
        dp[j][last] = 0;
      }
    }
    for (int j = 0; j <= k; ++j) {
      for (int last = 0; last < i; ++last) {
        int res = d[j][last] * 1ll * two[last] % mod;
        dp[j][last] = (dp[j][last] + res);
        if (dp[j][last] >= mod) dp[j][last] -= mod;
        dp[j + 1][i] =
            (dp[j + 1][i] + res * 1ll * (two[i - last] + mod - 1) % mod);
        if (dp[j + 1][i] >= mod) dp[j + 1][i] -= mod;
      }
    }
  }
  long long ans = 0;
  for (int i = 0; i <= n; ++i) {
    ans = (ans + dp[k][i]);
    if (ans >= mod) ans -= mod;
  }
  cout << ans << endl;
}
