#include <bits/stdc++.h>
using namespace std;
template <class TH>
void _dbg(const char *sdbg, TH h) {
  cerr << sdbg << " = " << h << endl;
}
template <class TH, class... TA>
void _dbg(const char *sdbg, TH h, TA... a) {
  while (*sdbg != ',') cerr << *sdbg++;
  cerr << " = " << h << ", ";
  _dbg(sdbg + 1, a...);
}
const int maxn = (1e2) + 7;
const int maxm = 1007;
const int maxk = 20;
const int inf = (1e9) + 7;
const long long LLinf = (1e18) + 7;
const long double eps = 1e-9;
const long long mod = 1e9 + 7;
long double dp[maxn * maxm][maxn];
int pos[maxm];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, m;
  cin >> n >> m;
  for (int i = 1; i <= n; i++) cin >> pos[i];
  if (m == 1) {
    cout << "1.00000000000000000";
    return 0;
  }
  dp[0][0] = 1.0;
  for (int j = 1; j <= n; j++) {
    for (int i = 1; i <= j * m; i++) {
      dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1];
      if (pos[j] != m) {
        if (i - pos[j] - 1 >= 0) dp[i][j] += dp[i - pos[j] - 1][j - 1];
        if (i - m - 1 >= 0) dp[i][j] -= dp[i - m - 1][j - 1];
      }
      if (i - pos[j] >= 0) dp[i][j] -= dp[i - pos[j]][j - 1];
    }
    if (j < n)
      for (int i = 1; i <= j * m; i++) {
        dp[i][j] /= (long double)(m - 1.0);
      }
  }
  long double res = 0.0;
  long long sum = 0;
  for (int i = 1; i <= n; i++) sum += pos[i];
  for (int i = 0; i < sum; i++) res += dp[i][n];
  res += 1.0;
  cout << fixed << setprecision(20) << res;
  return 0;
}
