#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const double EPS = 1e-13;
long long n, m, rnk[102], rsum;
double prob[102][100005];
double sm[100005];
double ans;
void solve() {
  scanf("%lld%lld", &n, &m);
  for (long long i = 1; i <= n; i++) {
    scanf("%lld", &rnk[i]);
    rsum += rnk[i];
  }
  prob[0][0] = 1;
  for (long long i = 1; i <= n; i++) {
    if (i == 1)
      sm[0] = 1.0;
    else
      sm[0] = 0.0;
    for (long long j = 1; j <= n * m; j++) sm[j] = sm[j - 1] + prob[i - 1][j];
    long long R = rnk[i];
    double tmp = 0.0;
    for (long long j = i; j <= n * m; j++) {
      prob[i][j] = sm[j - 1] - (j - m > 0 ? (sm[j - m - 1]) : 0) -
                   (j - R >= 0 ? prob[i - 1][j - R] : 0);
      prob[i][j] /= (m - 1);
      tmp += prob[i][j];
    }
  }
  for (long long i = n; i < rsum; i++) ans += prob[n][i];
  ans *= (m - 1);
  ans += 1.0 + EPS;
  printf("%.10f", ans);
  return;
}
signed main() {
  solve();
  return 0;
}
