#include <bits/stdc++.h>
#pragma warning(disable : 4996)
using namespace std;
int n, K;
long long Mod = 1000000007, D[2][501][501], po[510], Gap[501][501], Res;
int main() {
  scanf("%d%d", &n, &K);
  if (!K) {
    printf("1\n");
    return 0;
  }
  int i, j, k, pv = 0;
  po[0] = 1;
  for (i = 1; i <= n; i++) po[i] = po[i - 1] * 2 % Mod;
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= n; j++) {
      Gap[i][j] = po[j] * (po[i - j] - 1 + Mod) % Mod;
    }
  }
  for (i = 1; i <= n; i++) {
    pv = !pv;
    D[pv][1][i] = (po[i] - 1 + Mod) % Mod;
    for (j = 1; j < i; j++) {
      for (k = j; k < i; k++) {
        D[pv][j][k] = (D[pv][j][k] + D[!pv][j][k] * po[k]) % Mod;
        D[pv][j + 1][i] = (D[pv][j + 1][i] + D[!pv][j][k] * Gap[i][k]) % Mod;
        D[!pv][j][k] = 0;
      }
    }
  }
  for (i = 1; i <= n; i++) {
    Res = (Res + D[pv][K][i]) % Mod;
  }
  printf("%lld\n", Res);
}
