#include<stdio.h>

float a[30][30], v[30];
int n, k;

long long c;

void citire()
{
  scanf("%d%d",&n,&k);
  for (int i=1; i<=n; i++) scanf ("%f",&v[i]);
}

int main()
{
  int i,j;
  freopen("dezastru.in","r",stdin);
  freopen("dezastru.out","w",stdout);
  citire();
  c=1;
  for (i=1+k; i<=n; i++) c*=i;
  for (i=2; i<=(n-k); i++) c/=i;

  a[1][1]=v[1];
  for (i=2; i<=n; i++)
    a[1][i]=(a[0][i]+a[0][i-1])*v[1];
  for (i=2; i<=n; i++)
    for (j=1; j<=n; j++)
      a[i][j]=a[i-1][j]+a[i-1][j-1]*v[i];
  printf("%.6f",a[n][k]/c*2);
  return 0;
}