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


    if(p==k)
    {
        ss+=x;
        nn++;
    }else
    {
        for(char i=t+1;i<=n+p-k+1;++i)
            back(p+1,i,ve[i]*x);
    }

}
int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    for(char i=1;i<=n;i++)scanf("%lf",&ve[i]);

    if(k==0)printf("100");
    else
    {
        for(char i=1;i<=n-k+1;++i)
            back(1,i,ve[i]);
        ss=ss/nn;
        printf("%.6lf ",ss);
    }

    return 0;
}