#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
const int INF = 1000000007;
const int N = 100005;
double dp[101][N];
int a[N];
double sum[N];
int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  int mx = 0;
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
    mx += a[i];
  }
  if (m == 1) {
    puts("1");
    return 0;
  }
  for (int i = 1; i <= m; i++) dp[1][i] = 1.0;
  dp[1][a[1]] = 0.0;
  for (int i = 2; i <= n; i++) {
    for (int j = 1; j <= mx; j++) sum[j] = sum[j - 1] + dp[i - 1][j];
    for (int j = 1; j <= mx; j++) {
      if (j >= m)
        dp[i][j] = sum[j - 1] - sum[j - m - 1];
      else
        dp[i][j] = sum[j - 1];
      if (j > a[i]) dp[i][j] -= dp[i - 1][j - a[i]];
      dp[i][j] /= (m - 1);
    }
  }
  double ret = 0.0;
  for (int i = 1; i < mx; i++) ret += dp[n][i];
  printf("%.9lf\n", 1.0 + ret);
  return 0;
}
