#include <bits/stdc++.h>

using namespace std;

int s[26],n,m;
bool used[26];
float posibil[26],p,pn=1;


void afis(int k)
{
    for(int i=1;i<=k;++i)printf("%d ",s[i]);
    printf("\n");
}
void test(int k)
{
    float c=1;
    for(int i=1;i<=k;++i)
        c*=posibil[s[i]];

    p+=2*c/pn;
}
void bkt(int k)
{
    if(k == m+1)test(k-1);
    else
        for(int i=s[k-1]+1;i<=n;++i)
    {
        s[k]=i;
        bkt(k+1);
    }
}
void citire()
{
    scanf("%d %d",&n,&m);
    for(int i=1;i<=n;++i)
        scanf("%f",&posibil[i]);

    for(int i = n-m+1;i<=n;++i)
        pn*=i;
}
int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    citire();
    bkt(1);
    cout << setprecision(6) << fixed;
    printf("%f ",p);
    return 0;
}