#include <cstdio>

int x[32];
int N, K;
double a[32];
double dp[32][32];

void read()
{
	freopen("dezastru.in","r",stdin);
	scanf("%d %d\n", &N, &K);
	int i;
	for(i=1;i<=N;++i) scanf("%lf ", a+i);
}

void solve()
{
	int i, j;
	for(i=0;i<=N;++i) dp[i][0]=1;
	dp[1][1]=a[1];
	
	for(i=1;i<=N;++i)
		for(j=1;j<=i;++j)
			if(i==1 && j==1)continue;
			else dp[i][j]=dp[i-1][j]+dp[i-1][j-1]*a[i];
	
			
	/*
	for(i=0;i<=N;++i)
	{
		for(j=0;j<=i;++j)printf("%lf ", dp[i][j]);
		printf("\n");
	}
	*/
	printf("%.7lf\n", dp[N][K]/(double)3);
		
	
}

int main()
{
	freopen("dezastru.out","w",stdout);
	read();
	solve();
	return 0;
}