#include<cstdio>
#include<algorithm>
using namespace std;
int n,k,i,j;
double a[30][30],v[30],nr;
int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d %d",&n,&k); nr=1;
    for (i=1;i<=n;i++)
    scanf("%lf",&v[i]),a[i][0]=1;
    for (i=n-k+1;i<=n;i++) nr=nr*i;
    for (i=1;i<=k;i++) nr=nr/i;
    a[0][0]=1;
    for (i=1;i<=n;i++)
    for (j=1;j<=k;j++)
    a[i][j]=a[i-1][j]+(a[i-1][j-1]*v[i]);
    printf("%.6f",a[n][k]/nr);
    return 0;
}