#include<stdio.h>
double n,k,ve[27],ok=1,ss,nn;
int i;
double back(int p,int t,double x)
{

    int i;
    if(p==k)
    {
        ss+=x;
        nn++;
    }else
    {
        for(i=t+1;i<=n+p-k+1;i++)
            back(p+1,i,ve[i]*x);
    }
    return 0;
}
int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%lf%lf",&n,&k);
    for(i=1;i<=n;i++)scanf("%lf",&ve[i]);

    if(k==0)printf("100");
    else
    {
        for (i=1;i<=n-k+1;i++)
            back(1,i,ve[i]);
        ss=ss/nn;
        printf("%lf ",ss);
    }

    return 0;
}