#include<stdio.h>
#define M 26
int n,k,b[M][M];
double a[M][M],v[M];
int main()
{int i,j;
 FILE*f=fopen("dezastru.in","r");
 FILE*g=fopen("dezastru.out","w");
 fscanf(f,"%d %d",&n,&k);
 for(i=0;i<n;i++)
	fscanf(f,"%lf",&v[i]);
 a[0][1]=1;
 b[0][1]=1;
 for(i=1;i<=n;i++)
	for(j=1;j<=k+1;j++)
		{a[i][j]=a[i-1][j-1]*v[i-1]+a[i-1][j];
		 b[i][j]=b[i-1][j-1]+b[i-1][j];}
 fprintf(g,"%.6lf",a[n][k+1]/b[n][k+1]);
 fcloseall();
 return 0;
}