#include<stdio.h>

const int maxn = 100;

double s[maxn];
double prod;
double sum;
int nr;
int k;
int i;
int j;
int comb[maxn][maxn];
int a[maxn];
int n;


int bkt(int i)
{
        for(a[i] = a[i - 1] + 1;a[i] <= n; ++a[i])
        {
                if (i != k)
                {
                        prod *= s[a[i]];
                        bkt(i + 1);
                        prod /= s[a[i]];
                }
                else
                {
                        sum += prod * s[a[i]];
                        ++nr;
                }
        }
}

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",&s[i]);
        }

        prod = 1;
        bkt(1);

        printf("%lf",sum / nr);

        return 0;
}