#include <bits/stdc++.h>
using namespace std;
long long a[501];
long long b[501];
long long dp[2][501][501];
int main() {
  int n, k, m = 1000000007, i, j, l;
  long long ans = 0;
  scanf("%d %d", &n, &k);
  a[0] = 1;
  for (i = 1; i <= 500; i++) a[i] = a[i - 1] * 2 % m;
  for (i = 1; i <= 500; i++) {
    b[i] = a[i] - 1;
    if (b[i] < 0) b[i] += m;
  }
  dp[1][0][0] = 1;
  for (i = 1; i <= n; i++) {
    for (j = 0; j <= k; j++) {
      for (l = 0; l < i; l++) {
        if (dp[i & 1][j][l] == 0) continue;
        dp[(i + 1) & 1][j][l] =
            (dp[(i + 1) & 1][j][l] + dp[i & 1][j][l] * a[l] % m) % m;
        dp[(i + 1) & 1][j + 1][i] =
            (dp[(i + 1) & 1][j + 1][i] +
             dp[i & 1][j][l] * a[l] % m * b[i - l] % m) %
            m;
        dp[i & 1][j][l] = 0;
      }
    }
  }
  for (i = 0; i <= n; i++) ans += dp[(n + 1) & 1][k][i];
  ans %= m;
  printf("%I64d\n", ans);
  return 0;
}
