#include <bits/stdc++.h>
using namespace std;
const int N = 510, mod = 1e9 + 7;
int dp[N][N];
int power[N * N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, k;
  cin >> n >> k;
  dp[0][0] = 1;
  power[0] = 1;
  for (int i = 1; i < N * N; i++) power[i] = (1LL * power[i - 1] * 2) % mod;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= min(i, k); j++) {
      if (j == 0) {
        dp[i][j] = 1;
      } else {
        dp[i][j] += dp[i - 1][j];
        for (int lf = i; lf >= j; lf--) {
          long long w1 = power[(n - i) * (i - lf)];
          long long w2 =
              power[n - lf] + 1LL * (power[n - i] - 1) * (power[i - lf] - 1);
          w2 %= mod;
          w1 *= w2;
          w1 %= mod;
          w1 *= dp[lf - 1][j - 1];
          w1 %= mod;
          dp[i][j] = (dp[i][j] + w1) % mod;
        }
      }
    }
  }
  cout << dp[n][k] << endl;
}
