#include<cstdio>
using namespace std;
const int NMAX=30;
double v[NMAX],s;
int n,k,comb;
inline void combinari(int ind, int nr, double prod){
    if(ind>k){
        s+=prod;
        comb++;
        return ;
    }
    int m=n-(k-ind);
    for(int i=nr+1;i<=m;i++)
        combinari(ind+1, i, prod*v[i]);
}
int main(){
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d", &n, &k);
    for(int i=1;i<=n;i++)
        scanf("%lf", &v[i]);
    combinari(1, 0, 1.0);
    printf("%.6f", double(s/comb));
    return 0;
}