#include <bits/stdc++.h>
using namespace std;
int n, m, tot, a[110];
double dp[101][100001], sum[100001];
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]), tot += a[i];
  dp[0][0] = 1;
  sum[0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = i; j <= i * m; j++) {
      int L = max(i - 1, j - m), R = min((i - 1) * m, j - 1);
      dp[i][j] = sum[R] / (m - 1);
      if (L) dp[i][j] -= sum[L - 1] / (m - 1);
      if (L <= j - a[i] && j - a[i] <= R)
        dp[i][j] -= dp[i - 1][j - a[i]] / (m - 1);
    }
    sum[0] = 0;
    for (int j = 1; j <= i * m; j++) sum[j] = sum[j - 1] + dp[i][j];
  }
  double ans = 0;
  for (int i = n; i < tot; i++) ans += dp[n][i];
  printf("%.10lf\n", ans * (m - 1) + 1);
}
