#include <bits/stdc++.h>
using namespace std;
int n, m;
int r[105];
double D[105][105 * 1005];
int main() {
  scanf("%d %d", &n, &m);
  int inits = 0;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &r[i]);
    inits += r[i];
  }
  if (m == 1) {
    printf("%.11lf\n", 1.0);
    return 0;
  }
  D[0][0] = 1;
  for (int i = 1; i <= n; ++i) {
    double s = 0;
    for (int j = 0; j <= n * m; ++j) {
      D[i][j] += s - (j >= r[i] ? D[i - 1][j - r[i]] : 0);
      s += D[i - 1][j];
      if (j >= m) {
        s -= D[i - 1][j - m];
      }
      D[i][j] /= (m - 1);
    }
  }
  double p = 0;
  for (int j = 0; j < inits; ++j) {
    p += D[n][j];
  }
  printf("%.11lf\n", p * (m - 1) + 1);
  return 0;
}
