#include <fstream>
 
using namespace std;
 
int n, m, nrc, m1;
double a[28], sol;
 
inline void back(int k, int s, double sum)
{
    if (k == m1)
    {
        sol += sum;
        nrc++;
    }
    else
        for (int i=s + 1; i<=n-m+k; i++)
            back(k+1, i, sum*a[i]);
}
 
int main()
{
    ifstream f("dezastru.in");
    f>>n>>m;
    int i;
    for (i=1; i<=n; i++)
        f>>a[i];
    f.close();  
     
    m1 = m + 1;
    back(1, 0, 1);
     
    ofstream g("dezastru.out");
    g<<sol/nrc<<"\n";
    g.close();
     
    return 0;
}
 
/*#include <fstream>
#include <cstdio>
 
using namespace std;
 
int n, m, t[30][30], nrc, c[28];
double a[28], sol;
 
void Read()
{
    ifstream f("dezastru.in");
    f>>n>>m;
    int i;
    for (i=1; i<=n; i++)
        f>>a[i];
    f.close();  
}
 
void GenComb()
{
    int i, j;
    for (i=0; i<28; i++)
        t[i][i] = t[i][0] = 1;
    for(i=1; i<28; i++)
        for(j=1; j<i; j++)
            t[i][j] = t[i-1][j] + t[i-1][j-1];
}
 
inline void back(int k, int s, double sum)
{
    if (k-1 == m)
    {
        sol += sum;
    }
    else
    {
        int i;
        for (i=s + 1; i<=n-m+k; i++)
        {
            back(k+1, i, sum*a[i]);
        }
    }
}
 
void Solve()
{
    GenComb();
    nrc = t[n][m];
    back(1, 0, 1);
}
 
void Write()
{
    freopen("dezastru.out", "w", stdout);
    printf("%.6lf\n", sol/nrc);
}
 
int main()
{
    Read();
    Solve();
    Write();
    return 0;
}
 
*/