#include <bits/stdc++.h>
using namespace std;
int n, m, x[101];
double e[2][101 * 1001], sume[101 * 1001];
int main(int argc, const char* argv[]) {
  scanf("%d%d", &n, &m);
  for (int i = 0; i < n; i++) scanf("%d", x + i);
  if (m == 1) {
    puts("1.00");
    return 0;
  }
  int cur = 0, pre = 1;
  e[pre][0] = m - 1;
  for (int i = 0; i < n; i++) {
    cur = i % 2;
    pre = 1 - cur;
    sume[0] = e[pre][0];
    for (int j = 1; j <= (i + 1) * m; j++) sume[j] = sume[j - 1] + e[pre][j];
    for (int s = 0; s <= (i + 1) * m; s++) {
      int l, r;
      if (s >= 1) {
        l = max(s - m, 0), r = s - 1;
        if (l - 1 >= 0)
          e[cur][s] = sume[r] - sume[l - 1];
        else
          e[cur][s] = sume[r];
        if (l <= s - x[i] && s - x[i] <= r) e[cur][s] -= e[pre][s - x[i]];
        e[cur][s] /= (double)(m - 1.0);
      } else
        e[cur][s] = 0;
    }
  }
  int sum = 0;
  double ans = 0;
  for (int i = 0; i < n; i++) sum += x[i];
  for (int i = 0; i < sum; i++) ans += e[cur][i];
  printf("%.9lf\n", ans + 1.0);
  return 0;
}
