#include <bits/stdc++.h>
using namespace std;
int n, m;
int a[111];
long double cnt[100100];
long double presum[100100];
void compPreSum() {
  presum[0] = 0;
  for (int i = -1; ++i < n * m;) presum[i + 1] = presum[i] + cnt[i];
}
long double getSum(int l, int r) {
  l = max(l, 0);
  r = max(r, 0);
  return presum[r] - presum[l];
}
int main(void) {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  if (m == 1) {
    cout << 1;
    return 0;
  }
  int s = 0;
  for (int i = -1; ++i < n;) {
    cin >> a[i];
    s += a[i];
  }
  cnt[0] = 1;
  for (int comp = -1; ++comp < n;) {
    compPreSum();
    for (int i = n * m + 1; i--;) {
      cnt[i] = getSum(i - m, i);
      if (i >= a[comp]) cnt[i] -= cnt[i - a[comp]];
      cnt[i] /= m - 1;
    }
  }
  compPreSum();
  cout << fixed << setprecision(12) << getSum(0, s) * (m - 1) + 1;
  return 0;
}
