#include <stdio.h>
FILE*in = fopen("dezastru.in","r");
FILE*out= fopen("dezastru.out","w");
float p[30],rez,x;
long n,k,nr;
int main(){
    int i=0,j=0,c=0,l=0;

    fscanf(in,"%d%d",&n,&k);

    for(i=1;i<=n;i++)
       fscanf(in,"%f",&p[i]);


    for(j=1;j<n;j++){

       x=p[j];c=0;

       for(l=i=j+1;i<=n||c==k-1;)

	  if(c<k-1){x*=p[i];c++;i++;}

	  else{
	       rez+=x;
	       x=p[j];
	       i= ++l;
	       nr++;c=0;
	  }


    }

    fprintf(out,"%0.6f",rez/nr);
    return 0;
}