#include <bits/stdc++.h>

using namespace std;

#define _ ios_base::sync_with_stdio(0);cin.tie(0);
#define endl '\n'
#define f first
#define s second
#define pb push_back

typedef long long ll;
typedef pair<int, int> ii;

const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3fll;

const int MAX = 5e3+10, MOD = 1e9+7;
int dp[MAX][MAX];
int sulf[MAX][MAX];
int sulf_pa[MAX][MAX];

inline int multmod(int a, int b){
  return ll(a)*b % MOD;
}

int main(){ _
  int s, b; cin >> s >> b;
  for (int i = 1; i <= b; i++) dp[i][i] = 1;
  for (int j = 1; j <= b; j++) {
    for (int i = 1; i < j; i++) {
      // soma de PA da dp[x][j-i] para x \in {1, ..., i}
      // sum = (sum - sulf[j-i][i+1] + MOD) % MOD;
      int sum = sulf[j-i][1] - sulf[j-i][i+1];
      if(sum < 0) sum += MOD;

      // ans = (sulf_pa[j-i][1] - multmod(s-i, sum) + MOD) % MOD;
      int ans = sulf_pa[j-i][1] - multmod(s-i, sum);
      if(ans < 0) ans += MOD;

      // ans = (ans - sulf_pa[j-i][i+1] + MOD) % MOD;
      ans = ans - sulf_pa[j-i][i+1];
      if(ans < 0) ans += MOD;

      dp[j][i] = ans;
    }

    sulf[j][s] = dp[j][s];
    sulf_pa[j][s] = dp[j][s];
    for (int i = s-1; i > 0; i--) {
      // sulf[j][i] = (dp[j][i] + sulf[j][i+1]) % MOD;
      sulf[j][i] = dp[j][i] + sulf[j][i+1];
      if (sulf[j][i] >= MOD) sulf[j][i] -= MOD;

      // sulf_pa[j][i] = (multmod(dp[j][i], s-i+1) + sulf_pa[j][i+1]) % MOD ;
      sulf_pa[j][i] = multmod(dp[j][i], s-i+1) + sulf_pa[j][i+1];
      if(sulf_pa[j][i] >= MOD) sulf_pa[j][i] -= MOD;
    }
  }

  cout << dp[b][s] << endl;
  exit(0);
}