#include <bits/stdc++.h>
#define all(x) x.begin(), x.end()
#define pb push_back
#define eb emplace_back
#define rz resize
#define MP make_pair
#define MT make_tuple
#define IT iterator
#define fi first
#define se second
#define For(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define Rep(i, a, b) for (int i = (int)(a); i >= (int)(b); --i)
#define CLR(a, v) memset(a, v, sizeof(a))
#define CPY(a, b) memcpy(a, b, sizeof(a))
#define debug cerr << "ztxakking\n"
#define y0 ztxaknoi
#define y1 ztxakioi
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using uint = unsigned int;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pli = pair<ll, int>;
using pil = pair<int, ll>;
using vi = vector<int>;
template<typename T>
using V = vector<T>;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
template<const int MOD>
struct Modular {
  int val;
  Modular() = default;
  Modular(int x) : val(x) {}
  friend Modular operator + (Modular x, Modular y) { int res = x.val + y.val; if (res >= MOD) res -= MOD; return res; }
  friend Modular operator - (Modular x, Modular y) { int res = x.val - y.val; if (res < 0) res += MOD; return res; }
  friend Modular operator * (Modular x, Modular y) { int res = 1ll * x.val * y.val % MOD; return res; }
  Modular &operator += (Modular x) { *this = *this + x; return *this; }
  Modular &operator -= (Modular x) { *this = *this - x; return *this; }
  Modular &operator *= (Modular x) { *this = *this * x; return *this; }
  Modular &operator ++ () { *this = *this + 1; return *this; }
  Modular &operator ++ (int) { *this = *this + 1; return *this; }
  Modular &operator -- () { *this = *this - 1; return *this; }
  Modular &operator -- (int) { *this = *this - 1; return *this; }
  Modular operator - () { int res = val; if (res) res = MOD - res; return res; }
  static Modular pow(Modular x, ll p) {
    Modular ans(1);
    for (; p; p /= 2, x *= x) {
      if (p & 1) ans *= x;
    }
    return ans;
  }
  static Modular inv(Modular x) { return pow(x, MOD - 2); }
  friend Modular operator / (Modular x, Modular y) { return x * inv(y); }
  Modular &operator /= (Modular x) { *this = *this / x; return *this; }
  friend bool operator == (Modular x, Modular y) { return x.val == y.val; }
  friend bool operator != (Modular x, Modular y) { return x.val != y.val; }
  friend istream &operator >> (istream &in, Modular &item) { in >> item.val; return in; }
  friend ostream &operator << (ostream &out, Modular item) { out << item.val; return out; }
};
using mint = Modular<1000000007>;
const int N = 157, M = 1.2e4 + 7;
int n, m, L;
mint pw[M], w[M], fac[M], ifac[M];
V<mint> operator + (V<mint> a, V<mint> b) {
  int n = a.size() - 1, m = b.size() - 1;
  V<mint> c(max(n, m) + 1, 0);
  For(i, 0, n) c[i] += a[i];
  For(i, 0, m) c[i] += b[i];
  return c;
}
V<mint> operator - (V<mint> a, V<mint> b) {
  int n = a.size() - 1, m = b.size() - 1;
  V<mint> c(max(n, m) + 1, 0);
  For(i, 0, n) c[i] += a[i];
  For(i, 0, m) c[i] -= b[i];
  return c;
}
V<mint> operator * (V<mint> a, mint t) {
  V<mint> b = a;
  for (auto &i : b) i *= t;
  return b;
}
V<mint> operator * (V<mint> a, V<mint> b) {
  int n = a.size() - 1, m = b.size() - 1;
  V<mint> c(n + m + 1, 0);
  For(i, 0, n) For(j, 0, m) c[i + j] += a[i] * b[j];
  return c;
}
V<mint> Inv(V<mint> a) {
  int n = a.size() - 1;
  V<mint> b(n + 1, 0);
  mint t = mint().inv(a[0]);
  b[0] = t;
  For(i, 1, n) {
    For(j, 1, i) b[i] -= a[j] * b[i - j];
    b[i] *= t;
  }
  return b;
}
pair<V<mint>, V<mint>> solve(int l, int r) {
  if (l == r) {
    mint t = w[l] * ifac[l] * ifac[L - l];
    if (L - l & 1) t = -t;
    V<mint> f(1, t), g(2, 1);
    g[0] = -mint(l);
    return {f, g};
  }
  int mid = (l + r) / 2;
  auto [fl, gl] = solve(l, mid);
  auto [fr, gr] = solve(mid + 1, r);
  return {fl * gr + fr * gl, gl * gr};
}
int main() {
  ios::sync_with_stdio(0), cin.tie(0);
  cin >> n >> m, L = n * (n + 1) / 2;
  if (n == 2) return cout << "0\n", 0;
  For(y, 0, L) {
    pw[0] = 1;
    For(i, 1, L) pw[i] = pw[i - 1] * y;
    V<mint> a(n + 1, 0), e(1, 1);
    For(i, 1, n) a[i] = -pw[i * (i - 1) / 2];
    V<mint> b = Inv(e - a), c = a * b * b;
    c.rz(n + 1);
    V<mint> f = b * (m - 1) - c, g = b * (n - m) - c;
    mint ans = 0;
    For(l, 0, m - 1) For(r, 0, n - m) if (l || r) {
      ans -= (f[l] * b[r] + g[r] * b[l]) * pw[(m - l) * (n - m - r + 1) + (m - l) * (m - l - 1) / 2 + (n - m - r + 1) * (n - m - r) / 2];
    }
    w[y] = ans;
  }
  fac[0] = 1;
  For(i, 1, L) fac[i] = fac[i - 1] * i;
  ifac[L] = 1 / fac[L];
  Rep(i, L, 1) ifac[i - 1] = ifac[i] * i;
  auto [f, g] = solve(0, L);
  mint ans = 0;
  For(i, 0, L - 1) ans += mint().inv(L - i) * f[i];
  ans *= L * mint().inv(n - 1);
  cout << ans << '\n';
  return 0;
}