#include<cstdio>
float sol[30][30],a[30],c;
int n,i,j,k;
int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d %d",&n,&k);
	for(i=1;i<=n;i++) {scanf("%f",&a[i]); sol[i][0]=1;}
	sol[0][0]=1;
	for(i=1;i<=n;i++)
	{
		for(j=1;j<=k;j++)
		{
			sol[i][j]=sol[i-1][j]+sol[i-1][j-1]*a[i];
		}
	}
	c=1;
	for(i=k+1;i<=n;i++) c=c*i;
	for(i=1;i<=n-k;i++) c=c/i;
	printf("%f",sol[n][k]/c);
	return 0;
}