#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;

struct mod{
    int num;
    mod(){}
    mod(int num_):num(num_){
        if (num < 0) num += MOD;
    }
    mod operator+(mod a){
        int x = this->num;
        x += a.num;
        if (x >= MOD) return x-MOD;
        return x;
    }
    mod operator-(mod a){
        int x = this->num;
        x -= a.num;
        if (x < 0) return x+MOD;
        return x;
    }
    mod operator*(mod b){
        int x = this->num;
        return ll(x)*b.num % MOD;
    }
};

mod dp[MAX][MAX];
mod sulf[MAX][MAX];
mod sulf_pa[MAX][MAX];

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++) {
            mod ans = 0;
            mod summ = sulf[j-i][1];
            summ = summ - sulf[j-i][i+1];
            ans = sulf_pa[j-i][1] - (mod(s-i)*summ) - sulf_pa[j-i][i+1];
            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];

            sulf_pa[j][i] = (dp[j][i]*mod(s-i+1)) + sulf_pa[j][i+1];
        }
    }
    cout << dp[b][s].num << endl;
    exit(0);
}