#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 500 + 7;
const long long MOD = 1000000000 + 7;
long long dp[MAX_N][MAX_N];
long long pow2[MAX_N * MAX_N];
int main() {
  pow2[0] = 1;
  for (int i = 1; i < MAX_N * MAX_N; i++) pow2[i] = pow2[i - 1] * 2 % MOD;
  int N, K;
  scanf("%d%d", &N, &K);
  for (int i = 0; i <= N; i++) dp[i][0] = 1;
  for (int i = 1; i <= N; i++) {
    for (int j = 1; j <= min(i, K); j++) {
      for (int k = j - 1; k <= i; k++)
        dp[i][j] = (dp[i][j] + dp[k][j - 1] * (pow2[i - k] - 1) % MOD *
                                   pow2[k * (i - k)]) %
                   MOD;
    }
  }
  printf("%lld\n", dp[N][K]);
  return 0;
}
