#include <bits/stdc++.h>
using namespace std;
double ans = 0, sum = 0, f[55][55][55], c[55][55];
int n, m, a[55];
int main() {
  cin >> n >> m;
  f[0][0][0] = 1;
  for (register int i = 1; i <= m; ++i) scanf("%d", &a[i]), f[i][0][0] = 1;
  for (register int i = 0; i <= n; ++i) c[i][0] = 1;
  for (register int i = 1; i <= n; ++i)
    for (int j = 1; j <= n; ++j) c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
  for (register int i = 1; i <= m; ++i)
    for (register int j = 1; j <= n; ++j)
      for (register int l = 0; l <= n; ++l)
        for (register int k = 0; k <= min(j, l * a[i]); ++k)
          if (((!(k % a[i])) && (k / a[i] < l)) ||
              ((k % a[i]) && ((k / a[i]) + 1 < l)))
            f[i][j][l] += f[i - 1][j - k][l] * c[n - j + k][k];
          else
            for (register int w = 0; w <= l; ++w)
              f[i][j][l] += f[i - 1][j - k][w] * c[n - j + k][k];
  for (register int i = 1; i <= n; ++i)
    sum += f[m][n][i], ans += f[m][n][i] * i;
  printf("%.20lf\n", ans / sum);
}
