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

int main(){
    int i=0,j=0,c=0,l=0,q=0;

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

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


    for(i=1;i<=n-k+1;i++){

       x=p[i];

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

	  c=1;x=p[i];
	  for(q=j;q<=n && c<k;q++,c++) x*=p[q];

	  if(c>=k){rez+=x;nr++;}
       }


    }

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