#include <cstdio>
#include<algorithm>
using namespace std;
int main()
{
    FILE *f=freopen("dezastru.in","r",stdin),
    *g=freopen("dezastru.out","w",stdout);
	short i,n,k,st[25];long long cnt=0;
	double val[25],r=0,p;
	scanf("%hd %hd",&n,&k);
	for(i=0;i<n;++i)
	{
		st[i]=i;
		scanf("%lf",val+i);
	}
	do{ p=1;++cnt;
	for(i=0;i<k;++i)
		p*=val[st[i]];
	r+=p;
	}while(next_permutation(st,st+n));
    printf("%f",r/cnt);
}