#include <iostream>
using namespace std;

int k, m, n, a[99], b[99];
long double ans, totnum, num, p;


long double fact(long double n);
 
long double nCr(long double n, long double r)
{
    return fact(n) / (fact(r) * fact(n - r));
}
 
// Returns factorial of n
long double fact(long double n)
{
      if(n==0)
      return 1;
    long double res = 1;
    for (long double i = 2; i <= n; i++)
        res = res * i;
    return res;
}

void dfs(int nw, int sm) {
    if (nw > m) {
        if (sm ^ k)
            return;
        num = 1;
        p = 0;
        for (int i = 1; i <= m; ++i)
            num *= nCr(a[i], b[i]), p += (long double)b[i] / (a[i] - b[i] + 1);
        for (int i = 1; i < k; ++i)
            p *= i;
        if (p > 1)
            ans += num / totnum;
        else
            ans += num / totnum * p;
        return;
    }
    for (int i = 0; i + sm <= k && i <= a[nw]; ++i)
        b[nw] = i, dfs(nw + 1, sm + i);
}

int main() {
    cin >> k >> m;
    for (int i = 1; i <= m; ++i)
        cin >> a[i], n += a[i];
    totnum = nCr(n, k);
    dfs(1, 0);
    cout.precision(10);
    cout << fixed << ans << endl;
    return 0;
}