#include <cstdio>
#include <iomanip>
using namespace std;
FILE*f=fopen("dezastru.in","r");
FILE*g=fopen("dezastru.out","w");
int n,i,k,j;
double v[28],A[28][28];
int main()
{
    fscanf(f,"%d %d",&n,&k);
    for (i=1;i<=n;i++)
        fscanf(f,"%lf",&v[i]);
    for (i=0;i<=n;i++)
        A[i][0]=1;
    for (i=1;i<=n;i++)
    {
        for (j=1;j<=k && j<=i;j++)
        {
            A[i][j]=(A[i-1][j]*(i-j)+A[i-1][j-1]*v[i]*j)/i;
        }
    }
    fixed; setprecision(6);
    fprintf(g,"%lf",A[n][k]);
    return 0;
}