// #pragma GCC optimize("O3")
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <cmath>
#include <vector>
#include <cstdio>
#include <random>
#include <iomanip>
#include <cstring>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <unordered_map>
using namespace std;

#define PB emplace_back
#define int long long
#define ll long long
#define vi vector<int>
#define siz(a) ((int) ((a).size()))
#define rep(i, a, b) for (int i = (a); i <= (b); ++i)
#define per(i, a, b) for (int i = (a); i >= (b); --i)
void print(vi n) { rep(i, 0, siz(n) - 1) cerr << n[i] << " \n"[i == siz(n) - 1]; }

const int N = 50, mod = 1e9 + 7;
int a, b, f[N + 5][N + 5][N * N + 5][2];
bool vis[N + 5];

int qp(int n, int m = mod - 2) {
	int res = 1;
	for (; m; m >>= 1) {
		if (m & 1) res = res * n % mod;
		n = n * n % mod;
	}
	return res;
}
int S(int n) {return n * (n + 1) / 2 % mod;}
void add(int &n, int m) {
	n += m;
	if(n >= mod) n -= mod;
}
signed main() {
	// freopen(".in", "r", stdin);
	// freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> a >> b;
	b = a - b + 1;
	f[0][0][0][0] = mod - 1;
	int sum = a * (a + 1) / 2 % mod;
	rep(i, 0, a - 1) rep(j, 0, i) rep(k, 0, sum) rep(o, 0, 1) if(f[i][j][k][o]) {
		add(f[i + 1][j][k + i + 1 - j][o], f[i][j][k][o]);
		if(!o && i + 1 != b) add(f[i + 1][j][k + i + 1 - j][1], f[i][j][k][o]);
		if(i + 1 != b) add(f[i + 1][i + 1][k][o], mod - f[i][j][k][o]);
	}
	int ans = 0;
	rep(j, 0, a) rep(i, 0, sum - 1) if(f[a][j][i][1]) {
		// cout << j << ' ' << i << ' ' << f[a][j][i][1] << endl;
		(ans += f[a][j][i][1] * sum % mod * qp(sum - i + mod)) %= mod;
	}
	cout << ans * qp(a - 1) % mod;
	return cerr << endl << 1.0 * clock() / CLOCKS_PER_SEC << endl, 0;
}