#include<fstream>
using namespace std;
double a[26][26],v[26];
int c[26][26];
int main()
{int i,j,n,k;
ifstream in("dezastru.in");
in>>n>>k;
a[0][0]=1;
for(i=1;i<=n;i++) 
	{in>>v[i];
	a[i][0]=1;
	c[i][i]=c[i][0]=1;
	}
in.close();
for(i=1;i<=n;i++) 
	{for(j=1;j<=i;j++) 
		{c[i][j]=c[i-1][j-1]+c[i-1][j];
		a[i][j]=a[i-1][j]+v[i]*a[i-1][j-1];
		}
	c[i][i]=1;
	}
ofstream out("dezastru.out");
out<<a[n][k]/c[n][k]<<'\n';
out.close();
return 0;
}