#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O2,no-stack-protector,unroll-loops,fast-math")
const long long maxn = 1e5 + 10, maxm = 5e2 + 10, lg = 21, mod = 1e9 + 7,
                inf = 1e18;
long long n, m, s;
long double dp[101][maxn], ps[maxn];
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> n >> m;
  if (m == 1) return cout << 1, 0;
  dp[0][0] = m - 1;
  fill(ps, ps + maxn, m - 1);
  for (int i = 1; i <= n; i++) {
    long long x;
    cin >> x;
    s += x;
    for (int j = 1; j <= n * m; j++)
      dp[i][j] = (ps[j - 1] - (j - m - 1 >= 0 ? ps[j - m - 1] : 0) -
                  (j - x >= 0 ? dp[i - 1][j - x] : 0)) /
                 (m - 1);
    ps[0] = 0;
    for (int j = 1; j <= n * m; j++) ps[j] = ps[j - 1] + dp[i][j];
  }
  cout << fixed << setprecision(18) << ps[s - 1] + 1;
  return 0;
}
