/*#include <cstdio>

using namespace std;

FILE * f=fopen("dezastru.in","r");
FILE * g=fopen("dezastru.out","w");

int n,k;
double v[27],sum=0,nr=0;
void bk(int x,double s, int p)
{
    if(x>k)
    {
        sum += s;
        nr++;
    }
    else
    {
        for(int i = p+1; i <= n-k+x; ++i)
        {
            bk(x+1,s*v[i],i);
        }
    }
}
int main()
{
    fscanf(f,"%d%d",&n,&k);
    for(int i=1;i<=n;i++)
        fscanf(f,"%lf",&v[i]);
    bk(1,1,0);
    fprintf(g,"%.6f",sum/nr);
    //g<<fixed<<setprecision(6)<<sum/nr;
    return 0;
}*/
#include <iomanip>
#include <fstream>

using namespace std;

ifstream fi("dezastru.in");
ofstream fo("dezastru.out");

double sol=0;
double v[30];
int n,m;
int st[30];
int nrf = 0;

inline void bkt(int k, int s, double sum)
{
    if (k>m)
    {
        sol+=sum;
        nrf++;
        return;
    }
    for (register int i=s+1;i<=n-m+k;i++)
        bkt(k+1,i,sum*v[i]);
}
int main()
{
    fi>>n>>m;
    for(int i=1;i<=n;i++)
        fi>>v[i];
    bkt(1, 0, 1);
    fo<<setprecision(6)<<fixed<<(double)sol/nrf;
    return 0;
}