#include <cstdio>
using namespace std;
int c[59];
float prob[30];
int n,k,nr=0;
float sol=0;

inline float afisare()
{
    int j;
    float sol1=1;nr++;
    for(j=1;j<=k;j++) sol1*=prob[c[j]];
    return sol1*2;
}
inline void gencomb(int j,int i)
{
    if(j-1!=i)
    {
        for(int r=c[j-1]+1;r<=n-i+j;r++)
        {
            c[j]=r;
            gencomb(j+1,i);
        }
    }
    else sol+=afisare();
}
int main()
{   freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++) scanf("%f",&prob[i]);
    gencomb(1,k);
    printf("%.7f",sol/(nr*2));
}