#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int n = 0;
  bool b = 0;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') b = 1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    n = (n << 3) + (n << 1) + (c ^ 48), c = getchar();
  return b ? -n : n;
}
const int N = 1e2 + 7, M = 1e3 + 7;
int n, m, S;
int c[M];
double d, ans;
double f[N][N * M], sum[M];
signed main() {
  n = read(), m = read(), d = (1.0) / (m - 1);
  for (int i = 1; i <= n; i++) {
    c[i] = read();
    S += c[i];
  }
  f[0][0] = sum[0] = 1;
  for (int i = 1; i <= n; i++) {
    for (int j = i; j <= n * m; j++) {
      int L = max(i - 1, j - m), R = min((i - 1) * m, j - 1);
      f[i][j] = d * sum[R];
      if (L) f[i][j] -= d * sum[L - 1];
      if (L <= j - c[i] && j - c[i] <= R) f[i][j] -= d * f[i - 1][j - c[i]];
    }
    for (int j = 1; j <= i * m; j++) sum[j] = sum[j - 1] + f[i][j];
  }
  for (int j = n; j < S; j++) ans += f[n][j];
  printf("%.10lf", ans * (m - 1) + 1);
  return 0;
}
