#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007, maxn = 510;
int n, k;
long long dp[2][maxn][maxn], tvn[maxn], ans;
int main() {
  tvn[0] = 1;
  for (int i = 1; i < maxn; i++) tvn[i] = (tvn[i - 1] * 2) % mod;
  cin >> n >> k;
  dp[0][0][0] = 1;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j <= min(k, i + 1); j++) {
      for (int w = 0; w <= i + 1; w++) dp[(i & 1) ^ 1][j][w] = 0;
    }
    dp[(i & 1) ^ 1][0][0] = dp[i & 1][0][0];
    dp[(i & 1) ^ 1][1][i + 1] =
        (((tvn[i + 1] - 1 + mod) % mod) * dp[i & 1][0][0]) % mod;
    for (int j = 1; j <= min(k, i); j++) {
      for (int w = 1; w <= i; w++) {
        dp[(i & 1) ^ 1][j][w] =
            (dp[(i & 1) ^ 1][j][w] + ((tvn[w] * dp[i & 1][j][w]) % mod)) % mod;
        dp[(i & 1) ^ 1][j + 1][i + 1] =
            (dp[(i & 1) ^ 1][j + 1][i + 1] +
             ((((tvn[i + 1] - tvn[w] + mod) % mod) * dp[i & 1][j][w]) % mod)) %
            mod;
      }
    }
  }
  for (int i = 0; i <= n; i++) ans = (ans + dp[n & 1][k][i]) % mod;
  cout << ans;
  return 0;
}
