#include <bits/stdc++.h>
using namespace std;
ifstream fin("in.in");
ofstream fout("out.out");
const int N = 500 + 10, MOD = 1e9 + 7;
int n, m;
int dp[N][N], pw[((300) * (300))], res;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  if (!m) return (cout << 1), 0;
  pw[0] = 1;
  for (int i = 1; i < ((300) * (300)); i++) pw[i] = ((pw[i - 1] << 1) % MOD);
  dp[0][0] = 1;
  for (int i = 1; i <= m; i++)
    for (int j = 1; j <= n; j++)
      for (int k = 0; k < j; k++) {
        int s =
            ((pw[(j - k) * (n - j + 1)] - pw[(j - k) * (n - j)] + MOD) % MOD);
        dp[i][j] += (((1LL) * dp[i - 1][k] * s) % MOD);
        dp[i][j] %= MOD;
      }
  for (int i = 1; i <= n; i++) res += dp[m][i], res %= MOD;
  cout << res << endl;
  return 0;
}
