#include<fstream.h>

int j,n,k,t,u,i;

long double p,s,a[40],b[40][40];

int main()
{
 ifstream f("dezastru.in");
 ofstream g("dezastru.out");
 f>>n>>t;
 for(i=1;i<=n;i++)f>>a[i];
 for(i=0;i<=n;i++)b[i][0]=1;
 for(i=1;i<=n;i++)
  for(j=1;j<=i;j++)
   b[i][j]=b[i-1][j]+a[i]*b[i-1][j-1];
 s=b[n][t];
 for(i=t+1;i<=n;i++)s/=i;
 for(i=1;i<=n-t;i++)s*=i;
 g<<s;
 g.close();
 return 0;
 }