#include <cstdio>
#include <ctime>

using namespace std;

int n, k, C[26][26];
double prod, prob[26];

void read()
{
	freopen("dezastru.in", "r", stdin);
	freopen("dezastru.out", "w", stdout);
	scanf("%d %d", &n, &k);
	for(int i = 0; i < n; ++i)
		scanf("%lf", &prob[i]);
}

void back(int niv, int x, double aux)
{
	if(niv == k)
	{
		prod += aux;
		return;
	}
	for(int i = x; i+k <= n+niv; ++i)
	{
		back(niv + 1, i+1, aux * prob[i]);
	}
}

void solve()
{		
	int i, j;
	prod = 0;

	back(0, 0, 1.);
	
    for (i = 0; i <= n; ++i)
        for (C[i][0] = 1, j = 1; j <= i; ++j)
            C[i][j] = C[i-1][j]+C[i-1][j-1];
	
	printf("%lf", prod / (double)C[n][k]);
	/*do {
		prod += prob[nrp/k];
		
		for(int i = 0; i < perm.size(); ++i)
			cout << perm[i] << " ";
		cout << '\n';
		nrp++;
	} while(next_permutation(perm.begin(), perm.end()));*/
		
}

int main()
{
	//double tclock = clock();
	read();
	
	solve();
	//cout << (clock() - tclock) / CLOCKS_PER_SEC;
	return 0;
}