#include <bits/stdc++.h>
using namespace std;
const int N = 505;
const int Mod = 1e9 + 7;
int n, m;
int f[N][N];
int pw[N * N];
int powmod(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = res * a % Mod;
    a = a * a % Mod;
    b >>= 1;
  }
  return res;
}
long long countSeg(int x) { return x * (x + 1) / 2; }
int main() {
  pw[0] = 1;
  for (int i = 1; i < N * N; i++) {
    pw[i] = pw[i - 1] * 2 % Mod;
  }
  scanf("%d%d", &n, &m);
  if (m == 0) {
    puts("1");
    return 0;
  }
  memset(f, 0, sizeof(f));
  f[0][0] = 1;
  int ans = 0;
  for (int i = 1; i <= m; i++) {
    for (int j = 1; j <= n; j++) {
      for (int k = 0; k < j; k++) {
        long long out = countSeg(n - k) - countSeg(j - k) - countSeg(n - j);
        long long must = j - k;
        long long option = ((pw[out + must] - pw[out]) % Mod + Mod) % Mod;
        f[i][j] += f[i - 1][k] * option % Mod;
        f[i][j] %= Mod;
      }
      if (i == m) {
        ans += f[i][j];
        ans %= Mod;
      }
    }
  }
  printf("%d\n", ans);
}
