#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
const ll llinf = (1ll << 61) - 1;
const double eps = 1e-6, ldeps = 1e-9;
struct Dbg {
  static constexpr auto &os = cout;
  template <class C>
  static auto dud(C *x) -> decltype(os << *x, 0);
  template <class C>
  static char dud(...);
  template <class C>
  typename enable_if<sizeof dud<C>(0) != 1, Dbg &>::type operator<<(
      const C &x) {
    os << x;
    return *this;
  }
  template <class C>
  Dbg &dump(C b, C e) {
    *this << "\n[";
    int cur = 0;
    for (C i = b; i != e and cur < 26; i++, cur++)
      *this << ", " + 2 * (i == b) << *i;
    return *this << ']';
  }
  template <class C>
  typename enable_if<sizeof dud<C>(0) == 1, Dbg &>::type operator<<(
      const C &x) {
    return dump(begin(x), end(x));
  }
  template <class C, size_t X>
  typename enable_if<sizeof(C) != 1, Dbg &>::type operator<<(C (&x)[X]) {
    return dump(begin(x), end(x));
  }
  template <class B, class C>
  Dbg &operator<<(const pair<B, C> &x) {
    return *this << '{' << x.first << ", " << x.second << '}';
  }
  template <class C, size_t Y>
  struct TP {
    void operator()(Dbg &os, const C &t) {
      TP<C, Y - 1>()(os, t);
      os << ", " << get<Y - 1>(t);
    }
  };
  template <class C>
  struct TP<C, 1> {
    void operator()(Dbg &os, const C &t) { os << get<0>(t); }
  };
  template <class... C>
  Dbg &operator<<(const tuple<C...> &t) {
    os << '{', TP<decltype(t), sizeof...(C)>()(*this, t);
    return *this << '}';
  }
  Dbg &operator<<(ostream &(*x)(std::ostream &)) {
    os << x;
    return *this;
  }
} dbg;
void err(istringstream *iss) { delete iss; }
template <class C, class... Args>
void err(istringstream *iss, const C &val, const Args &...args) {
  string name;
  *iss >> name;
  if (name.back() == ',') name.pop_back();
  dbg << name << " = " << val << "; ", err(iss, args...);
}
int T, Q, cn;
const int inf = 1000000007, mxn = 105, mxm = 1005, mxs = 101005;
int n, m, x[mxn];
double dp[2][mxs], cs[mxs];
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(NULL);
  cout.precision(11);
  cin >> n >> m;
  if (m == 1) return cout << "1\n", 0;
  int score = 0;
  for (int i = 1; i <= n; i++) {
    cin >> x[i], score += x[i];
  }
  for (int i = 0; i < score; i++) {
    dp[(n + 1) & 1][i] = 1;
  }
  for (int i = n; i >= 1; i--) {
    for (int j = 1; j <= mxs - 1; j++) {
      cs[j] = cs[j - 1] + dp[(i + 1) & 1][j];
    }
    for (int j = 0; j < mxs - 1; j++) {
      dp[(i)&1][j] = (cs[j + x[i] - 1] - cs[j] + cs[min(j + m, mxs - 1)] -
                      cs[min(j + x[i], mxs - 1)]) /
                     (m - 1);
    }
  }
  cout << dp[(1) & 1][0] * (m - 1) + 1 << '\n';
}
