#include<cstdio>
#define l(Q,R,T) freopen("dezastru.Q","R",T);
int n,k,prod=0;double a[30],sum=0;void b(int x,double q,int d){int i;if (x==k+1){sum+=q;prod++;return;}int r=n+x-k;for(i=d+1;i<=r;++i)b(x+1,q*a[i],i);}int main(){l(in,r,stdin)l(out,w,stdout)scanf("%d%d",&n,&k);for(int i=1;i<=n;++i)scanf("%lf",&a[i]);b(1,1,0);printf("%lf",sum/prod);return 0;}