#include<cstdio>
int n,k,aux;float v[26];bool matrix[26][26];
void get()
{
	freopen("dezastru.in","r",stdin);
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;i++)
		scanf("%f",&v[i]);
	aux=n-k+1;
}
void prepare()
{
	int i,j;
	for(i=1;i<=aux;i++)
	{
		for(j=n;j>aux;j--)
			matrix[i][j]=1;
		matrix[i][aux-i+1]=1;
	}
}
void fight()
{
	freopen("dezastru.out","w",stdout);
	int i,j,m,index=aux;float ax=1,s=0;
	for(i=1;i<=aux;i++,ax=1)
	{
		for(j=n;j>0;j--)
			if(matrix[i][j])
				ax*=v[j];
		s+=ax;
		for(j=n,ax=1;j>aux-i+1;j--,ax=1)
			if(!matrix[i][j-1])
			{
				matrix[i][j-1]=1,matrix[i][j]=0,j=n+1;
				for(m=n;m>0;m--)
					if(matrix[i][m])
						ax*=v[m];
				s+=ax;++index;
			}
	}
	printf("%f",s/index);
}
int main()
{
	get();
	prepare();
	fight();
}