#include <cstdio>
#include <bitset>

FILE *in,*out;

using namespace std;

int h,pasi = 0,n;

int sol[26];
double p[26], s = 0;
bitset <26> verifica;

void explore(int k)
{
    if(k == h)
    {
        pasi ++;
        double r = 1;
        for(int i = 0; i < h; i ++)
        {
            r = r * p[sol[i]];
        }
        s += r;
    } else {
        for(int i = 0; i < n; i++) {
            if(verifica[i] == 0) {
                verifica[i] = 1;
                sol[k] = i;
                explore(k+1);
                verifica[i] = 0;
            }
        }
    }
}

int main()
{
    in = fopen("dezastru.in","r");
    out = fopen("dezastru.out","w");
    fscanf(in, "%d %d", &n, &h);
    for(int i = 0; i < n; i++) {
        fscanf(in, "%lf", &p[i]);
    }

    explore(0);
    double rez = s/pasi;
    fprintf(out,"%.6lf",rez);
    return 0;
}