#include<stdio.h>
#define d 26
int n,K,Nr,i;
double p,v[d];
void Back(int k, int last, double s){
    if(k == K + 1){
        p+=s;
		++Nr;
		return;
	}
	else
		for(int i=last+1; i<n-K+k+1; ++i)
			Back(k+1, i, s*v[i]);
}
int main(){
	freopen("dezastru.in", "r", stdin);
	freopen("dezastru.out", "w", stdout);
	scanf("%d %d",&n,&K);
    for(i=1;i<=n;++i)
		scanf("%lf",v+i);
	Back(1,0,1);
	p/=Nr;
	printf("%.6lf",p);
	fclose(stdin);
	fclose(stdout);
	return 0;
}