#include <bits/stdc++.h>
#define cs const
#define pb push_back
using namespace std;

int k, m, n;
double ans; 
double calc(vector <int> c, vector <int> d) {
    double ways = 1; 
    // prod C(ci, di)
    for(int i = 0; i < m; i++) {
        for(int j = 0; j < d[i]; j++)
            ways = ways * (c[i] - j) / (j + 1);
    }
    double cc = 0; 
    for(int i = 0; i < m; i++) if(d[i]) {
        double res = ways * d[i] / (c[i] - d[i] + 1); 
        cc = cc + res; 
    }
    for(int i = 1; i < k; i++) cc = cc * i; 
    return min(ways, cc);
    
}
void dfs(int u, int k, cs vector <int> & c, vector <int> d) {
    if(u == m) {
        if(k == 0) {
            ans += calc(c, d);
            // for(int x : d) cout << x << ' ';cout<<endl;
            // cerr << ans << endl;
        }
        return;
    }
    for(int i = 0; i <= min(k, c[u]); i++) {
        d[u] = i; 
        dfs(u + 1, k - i, c, d);
    }
}
int main() {
    #ifdef zqj
    freopen("1.in","r",stdin);
    #endif
    cin >> k >> m;
    vector <int> c(m), d(m);
    for(int i = 0; i < m; i++) 
        cin >> c[i], n += c[i];
    dfs(0, k, c, d);
    double all = 1;
    for(int i = 0; i < k; i++)  
        all = all * (n - i) / (i + 1);
    // cerr << all << endl;
    printf("%.12lf\n", ans / all);
    return 0; 
}