#include <bits/stdc++.h>
using namespace std;
const int N = 105;
const int M = 1005;
int n, m, my_pts, L, R;
int c[N];
double d, ans;
double dp[N][N * M], sum[N * M];
int main() {
  cin >> n >> m;
  d = 1.0 / (m - 1);
  for (int i = 1; i <= n; i++) cin >> c[i], my_pts += c[i];
  dp[0][0] = sum[0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = i; j <= i * m; j++) {
      L = max(i - 1, j - m), R = min((i - 1) * m, j - 1);
      dp[i][j] = d * sum[R];
      if (L) dp[i][j] -= d * sum[L - 1];
      if (L <= j - c[i] && j - c[i] <= R) dp[i][j] -= d * dp[i - 1][j - c[i]];
    }
    for (int j = 1; j <= i * m; j++) sum[j] = sum[j - 1] + dp[i][j];
  }
  for (int j = n; j < my_pts; j++) ans += dp[n][j];
  printf("%.12lf\n", ans * (m - 1) + 1);
}
