#include <bits/stdc++.h>

#define INF 10000000000000
#define MOD 1000000007
#define MaxN 26

using namespace std;

int N,K,comb;
double v[MaxN],ans;

void Back(int lvl,int last,double val)
{
    if(lvl == K)
    {
        comb++;
        ans+=val;
        return;
    }
    for(int i=last;i<=N-K+lvl+1;i++)
        Back(lvl+1,i+1,val*v[i]);
}

FILE *IN = fopen("dezastru.in","r");
FILE *OUT = fopen("dezastru.out","w");

int main()
{
    fscanf(IN,"%d %d",&N,&K);
    for(int i=1;i<=N;i++)
        fscanf(IN,"%lf",&v[i]);
    Back(0,1,1);
    fprintf(OUT,"%.8f",ans/comb);
    return 0;
}