#include <cstdio>
using namespace std;

double s[25][25];

double comb(int n,int k)
{
    int i;
    double p;
    for(i=n-k+1;i<=n;++i) p*=i;
    for(i=2;i<=k;++i) p/=i;
    return p;
}

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    int i,j,n,k;
    double p1,p[25];
    scanf("%d%d",&n,&k);
    for(i=0;i<n;++i) scanf("%lf",&p[i]);
    for(i=0;i<n;++i) s[i][0]=1;
    s[0][1]=p[0];
    for(i=1;i<n;++i)
    {
        for(j=1;j<k;++j)
        {
            s[i][j]=s[i-1][j]+p[i]*s[i-1][j-1];
        }
    }
    printf("%.6lf",s[n-1][k-1]/comb(n,k));
    return 0;
}