#include <bits/stdc++.h>
using namespace std;
long long mod = 1000000007;
long long dp[501][501];
long long p2[1000001];
int main() {
  int num, gen;
  scanf("%d%d", &num, &gen);
  p2[0] = 1;
  for (int i = 1; i <= 1000000; i++) p2[i] = p2[i - 1] * 2 % mod;
  dp[0][0] = 1;
  for (int i = 0; i < num; i++) {
    for (int j = 1; j <= num - i; j++) {
      long long coef = (p2[j] + mod - 1) * p2[i * j] % mod;
      for (int k = 0; k < gen; k++) {
        dp[i + j][k + 1] += dp[i][k] * coef;
        dp[i + j][k + 1] %= mod;
      }
    }
  }
  long long ans = 0;
  for (int i = 0; i <= num; i++) {
    ans += dp[i][gen] * p2[i * (num - i)];
    ans %= mod;
  }
  printf("%I64d\n", ans);
}
