#include <bits/stdc++.h>
using namespace std;
const int MAXN = 503;
const int mod = 1e9 + 7;
int dp[MAXN][MAXN];
int p[MAXN * MAXN];
int n, k;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> k;
  p[0] = 1;
  for (int i = 1; i <= n * n; i++) {
    p[i] = 2LL * p[i - 1] % mod;
  }
  dp[0][0] = 1;
  for (int j = 0; j <= k; j++) {
    for (int i = 1; i <= n; i++) {
      if (j == 0) {
        dp[i][j] = 1;
        continue;
      }
      for (int d = 1; d <= i; d++) {
        int c = (p[i - d + 1] - 1 + mod) % mod;
        int t = p[(i - d + 1) * (d - 1)];
        int m = (long long)t * (long long)c % mod;
        m = (long long)m * (long long)dp[d - 1][j - 1] % mod;
        dp[i][j] += m;
        dp[i][j] %= mod;
      }
    }
  }
  cout << dp[n][k] % mod << '\n';
}
