#include <bits/stdc++.h>
using namespace std;
const int maxn = 5e2 + 20;
const int maxt = maxn * maxn;
const long long mod = 1e9 + 7;
long long dp[maxn][maxn], t[maxt];
int main() {
  t[0] = 1;
  for (int i = 1; i < maxt; i++) t[i] = (t[i - 1] * 2) % mod;
  int n, k;
  cin >> n >> k;
  dp[0][0] = 1;
  for (int i = 1; i <= n; i++)
    for (int j = 0; j <= i; j++)
      for (int k = 0; k < i; k++) {
        if (!j)
          dp[i][j] = 1;
        else
          dp[i][j] +=
              (((t[k + 1] - 1 + mod) * t[(i - k - 1) * (k + 1)]) % mod) *
              dp[i - k - 1][j - 1];
        dp[i][j] %= mod;
      }
  cout << dp[n][k] << endl;
}
