#include <bits/stdc++.h>
using namespace std;
const int maxn = 103, maxm = 1003;
int m, n, ar[maxn];
double dp[maxn * maxm], pre[maxn * maxm];
int main() {
  scanf("%d%d", &n, &m);
  if (m == 1) {
    printf("1");
    return 0;
  }
  int sum = 0;
  for (int i = 1; i <= n; i++) scanf("%d", &ar[i]), sum += ar[i];
  dp[0] = 1;
  for (int i = 1; i <= n; i++) {
    pre[0] = dp[0];
    for (int j = 1; j <= i * m; j++) pre[j] = pre[j - 1] + dp[j];
    for (int j = i * m; j; j--) {
      dp[j] = pre[j - 1];
      if (j - m - 1 >= 0) dp[j] -= pre[j - m - 1];
      if (j - ar[i] >= 0) dp[j] -= dp[j - ar[i]];
      dp[j] /= m - 1;
    }
    dp[0] = 0;
  }
  double ans = 0;
  for (int i = 0; i < sum; i++) ans += dp[i];
  printf("%.10lf\n", 1 + ans * (m - 1));
  return 0;
}
