#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ldb;
typedef pair<int,int> pii;
const int MAXD = 10 + 5;

ldb C(int n,int m)
{
	ldb res = 0;
	for(int i=0; i<m; ++i)
		res = res + logl(n-i) - logl(i+1);
	return res;
}

int d, m;
int a[MAXD];

ldb ans = 0, tot = 0;

int use[MAXD];
void dfs(int dep,int has)
{
	if(dep>m)
	{
		if(has != d) return;
		
		ldb A = 0;
		for(int i=1; i<=m; ++i)
			A += C(a[i], use[i]);
		
		ldb B = 0;
		for(int i=1; i<=m; ++i) if(use[i])
		{
			B += (ldb)use[i] / (a[i] - use[i] + 1);
		}
		
		B = logl(B);
		B += A;
		for(int i=1; i<d; ++i)
			B += logl(i);
		
		ans += expl(min(A,B) - tot);
		
		return;
	}
	for(int i=0; has+i<=d && i<=a[dep]; ++i)
		use[dep] = i, dfs(dep+1, has+i);
}

int main(void)
{
	scanf("%d%d",&d,&m);
	for(int i=1; i<=m; ++i)
		scanf("%d",&a[i]);
	int n = accumulate(a+1,a+m+1,0);
	
	tot = C(n, d);
	dfs(1,0);
	
	printf("%.20lf\n",(double)ans);
	return 0;
}