#include <bits/stdc++.h>
using namespace std;
const int N = 110;
const int M = 1010;
double f[N][M * N], sum[N * M];
int a[N], n, m, s;
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; i++) {
    scanf("%d", a + i);
    s += a[i];
  }
  f[0][0] = 1;
  sum[0] = 1.0;
  for (int j = 1; j < s; j++) {
    sum[j] = sum[j - 1] + f[0][j];
  }
  for (int i = 1; i <= n; i++) {
    for (int j = i; j < s; j++) {
      int l = min(m, j - i + 1);
      int r = max(-1, j - l - 1);
      double tmp = sum[j - 1] - ((r == -1) ? 0 : sum[r]);
      if (l < a[i]) {
        f[i][j] = tmp / (m - 1);
      } else {
        f[i][j] = (tmp - f[i - 1][j - a[i]]) / (m - 1);
      }
    }
    sum[0] = 0;
    for (int j = 1; j < s; j++) {
      sum[j] = sum[j - 1] + f[i][j];
    }
  }
  double ans = 0;
  for (int i = n; i < s; i++) {
    ans += f[n][i];
  }
  printf("%.10f\n", 1 + ans * (m - 1));
  return 0;
}
