#include<stdio.h>

int n,m;
long long nr = 0;
double a[26], s;

void comb() {
    for (long long  i = 0; i < (1 << n ); i++) {
        int nr1 = 0, p2 = 1;
        for (int j = 0; j < n; j++, p2 <<= 1)
            if (i & p2)
                nr1++;
        if (nr1 == m) {
            double t = 1;
            p2 = 1;
            for(int j = 0; j < n; j++, p2 <<= 1)
                if (i & p2)
                    t *= a[j];
            s += t;
            nr++;
        }
    }
}
void citire() {
    freopen("dezastru.in","r",stdin);
    scanf("%d %d", &n,&m);
    for(int i = 0; i < n; i++)
        scanf("%lf", &a[i]);
}


int main() {
    citire();
    comb();
    freopen("dezastru.out","w",stdout);
    printf("%lf", s / nr);
    return 0;
}