#include <cstdio>

using namespace std;

int st[30],n,k,p;
float s,b[30];

void abc()
{
    float w=1;
    for(int i=1;i<=k;i++)w*=b[st[i]];
    s+=(2.0*w)/float(p);
}
void back(int x)
{
    if(x>k)abc();
    else for(int i=x;i<=n;i++)
    {
        st[x]=i;
        if(st[x]>st[x-1])
        back(x+1);
    }
}

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    p=1;
    for(int i=n-k+1;i<=n;i++)p=p*1LL*i;
    for(int i=1;i<=n;i++)scanf("%f",&b[i]);
    back(1);
    printf("%.6f",s);
    return 0;
}