#include <cstdio>

using namespace std;

int st[26],n,k,p;
float s,b[26];

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)
{
    for(int i=st[x-1]+1;i<=n;i++)
    {
        st[x]=i;
        if(x==k)abc();
        else back(x+1);
    }
}
unsigned long long comb(int n,int k)
{
    unsigned long long n2,nf,ns,j;
    n2=n-k;
    nf=ns=1;
    if(n==k)
    return 1;
    else
    {
    if(n2>=k)
    {
    for(j=n2+1;j<=n;j++)
    nf=nf*j;
    for(j=2;j<=k;j++)
    ns=ns*j;
    return (nf/ns)%30103;
    }
    else
    {
    for(j=k+1;j<=n;j++)
    nf=nf*j;
    for(j=2;j<=n2;j++)
    ns=ns*j;
    return (nf/ns)%30103;
    }
    }
}

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    p=2*1LL*comb(n,k);
    for(int i=1;i<=n;i++)scanf("%f",&b[i]);
    back(1);
    printf("%.6f",s);
    return 0;
}