#include <bits/stdc++.h>

#define INF 10000000000000
#define MOD 1000000007
#define MaxN 26

using namespace std;

int N,K,comb;
double fact[MaxN];
double v[MaxN],ans,d[MaxN][MaxN];

FILE *IN = fopen("dezastru.in","r");
FILE *OUT = fopen("dezastru.out","w");

int main()
{
    fact[0]=1;
    fscanf(IN,"%d %d",&N,&K);
    for(int i=1;i<=N;i++)
        fscanf(IN,"%lf",&v[i]),fact[i]=fact[i-1]*i;
    d[0][0]=1;
    for(int i=1;i<=N;i++)
    {
        d[i][0]=1;
        for(int j=1;j<=i;j++)
            d[i][j] = d[i-1][j]+v[i]*d[i-1][j-1];
    }
    fprintf(OUT,"%.8f",d[N][K]*fact[K]*fact[N-K]/fact[N]);
    return 0;
}