#include <bits/stdc++.h>

using namespace std;

// #define LOCK_GETCHAR
// #define USE_INT_128

#if __cplusplus < 201400
#warning "Please use c++14 or higher."
#define CONSTEXPR_FUNC
#define ENABLE_IF_INT
#else
#define CONSTEXPR_FUNC constexpr
#define ENABLE_IF_INT , enable_if_t<_is_integer<T>, int> = 0
template <class T> constexpr bool _is_integer = numeric_limits<T>::is_integer;
template <> constexpr bool _is_integer<bool> = false;
template <> constexpr bool _is_integer<char> = false;
#ifdef USE_INT_128
template <> constexpr bool _is_integer<__int128> = true;
template <> constexpr bool _is_integer<__uint128_t> = true;
#endif
#endif

#if !defined(_WIN32) && !defined(__CYGWIN__) && !defined(LOCK_GETCHAR)
#define getchar getchar_unlocked
#endif

#define il inline
#define mkp make_pair
#define fi first
#define se second
#define ssz(x) (signed((x).size()))
#define _loop_i_t(j, k) make_signed_t<decltype((j) - (k))>
#define For(i, j, k) for (_loop_i_t(j, k) i = (j); i <= (k); ++i) // NOLINT
#define ForDown(i, j, k)                                                       \
  for (_loop_i_t(j, k) i = (j); i >= decltype(i)(k); --i) // NOLINT
#define eb emplace_back
#ifndef ONLINE_JUDGE
#define FileIO(filename)                                                       \
  freopen(filename ".in", "r", stdin);                                         \
  freopen(filename ".out", "w", stdout)
#else
#define FileIO(filename) void(0)
#endif

using ll = long long;
using uint = unsigned int;
using ull = unsigned long long;
using db = double;
using ldb = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#ifdef USE_INT_128
using lll = __int128_t;
using ulll = __uint128_t;
#endif

// clang-format off
CONSTEXPR_FUNC il ll qpow(ll x, ull y, ll mod){ ll ans = 1; x %= mod; while (y) { if (y & 1) (ans *= x) %= mod; (x *= x) %= mod; y >>= 1; } return ans; }
template<typename T> CONSTEXPR_FUNC il void cmin(T & x, const T &y){ x = min(x, y); }
template<typename T> CONSTEXPR_FUNC il void cmax(T & x, const T &y){ x = max(x, y); }
template<typename T ENABLE_IF_INT> il void read(T &x){ x = 0; int f = 1; int c = getchar(); while (!isdigit(c)) { if (c == '-') f = -1; c = getchar(); } while (isdigit(c)) { x = x * 10 + (c - '0'); c = getchar(); } x *= f; }
template<typename T, typename ... Args> il void read(T &x, Args &... y){ read(x), read(y...); }
// clang-format on

// File head end

namespace {
constexpr ll MAXN = 152, MAXM = MAXN * MAXN / 2, mod = 1e9 + 7;
int n, m, tot;
ll inv[MAXM], f[MAXN][MAXM][2], g[MAXN][MAXM][2], f2[MAXM][MAXN][2],
    g2[MAXM][MAXN][2], ans[MAXM], coef[MAXM];
ll pw[MAXM][MAXN], tmp[MAXM];
il int calc(int x) { return x * (x + 1) / 2; }
void lagrange() {
  vector<ll> F = {1}, tmp;
  For(i, 0, tot) {
    tmp.resize(i + 2), copy(F.begin(), F.end(), tmp.begin() + 1), tmp[0] = 0;
    For(j, 0, i)(tmp[j] += F[j] * (mod - i)) %= mod;
    F.swap(tmp);
  }
  For(i, 0, tot) {
    ll mul = ans[i];
    For(j, 0, i - 1)(mul *= inv[i - j]) %= mod;
    For(j, i + 1, tot)(mul *= mod - inv[j - i]) %= mod;
    static vector<ll> G(ssz(F));
    copy(F.begin(), F.end(), G.begin());
    ForDown(j, tot + 1, 1) {
      (coef[j - 1] += G[j] * mul) %= mod;
      (G[j - 1] += G[j] * i) %= mod, G[j] = 0;
    }
    assert(!G[0]);
  }
}
void Main() {
  read(n, m);
  inv[1] = 1, tot = n * (n + 1) / 2;
  For(i, 0, tot) { For(j, 0, n) pw[i][j] = qpow(i, calc(j), mod); }
  For(i, 2, tot) inv[i] = inv[mod % i] * (mod - mod / i) % mod;
  ll sum = 0;
  For(i, 0, tot) f[m][i][0] = 1;
  ForDown(i, m - 1, 1) {
    For(j, i + 1, m) For(k, 0, tot) {
      int o = pw[k][j - i - 1];
      (f[i][k][0] += mod - f[j][k][0] * o % mod) %= mod;
      (f[i][k][1] += mod - f[j][k][1] * o % mod) %= mod;
      (f[i][k][1] += mod - f[j][k][0] * (j - i - 1) % mod * o % mod) %= mod;
    }
  }

  For(i, 0, tot) g[m][i][0] = 1;
  For(i, m + 1, n) {
    For(j, m, i - 1) For(k, 0, tot) {
      int o = pw[k][i - j - 1];
      (g[i][k][0] += mod - g[j][k][0] * o % mod) %= mod;
      (g[i][k][1] += mod - g[j][k][1] * o % mod) %= mod;
      (g[i][k][1] += mod - g[j][k][0] * (i - j - 1) % mod * o % mod) %= mod;
    }
  }

  For(i, 1, m) For(k, 0, tot) {
    ll u = f[i][k][0], v = f[i][k][1], o = pw[k][i - 1];
    f[i][k][0] = u * o % mod;
    f[i][k][1] = (u * (i - 1) + v) % mod * o % mod;
  }
  For(i, m, n) For(k, 0, tot) {
    ll u = g[i][k][0], v = g[i][k][1], o = pw[k][n - i];
    g[i][k][0] = u * o % mod;
    g[i][k][1] = (u * (n - i) + v) % mod * o % mod;
  }

  cerr << "OK1 " << 1. * clock() / CLOCKS_PER_SEC << '\n';

  For(i, 1, m) For(j, 0, tot) For(k, 0, 1) f2[j][i][k] = f[i][j][k];
  For(i, m, n) For(j, 0, tot) For(k, 0, 1) g2[j][i][k] = g[i][j][k];

  For(k, 0, tot) {
    tmp[0] = 1;
    For(i, 1, tot) tmp[i] = tmp[i - 1] * k % mod;
    For(i, 1, m) For(j, m, n) if (i != m || j != m) {
      ll mul = tmp[i * (n - j + 1)];
      For(o, 0, 1)(ans[k] += f2[k][i][o] * g2[k][j][o ^ 1] % mod * mul) %= mod;
    }
  }

  cerr << "OK2 " << 1. * clock() / CLOCKS_PER_SEC << '\n';

  lagrange();
  For(i, 0, tot)(sum += mod - coef[i] * inv[tot - i] % mod) %= mod;
  cout << sum * tot % mod * inv[n - 1] % mod << '\n';
}
} // namespace

signed main() { return Main(), 0; }