#include <cstdio>
#define M 26
#define ll long long
using namespace std;
float a[M][M],pi[M];
ll b[M][M];
int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    int n,i,j,k;
    scanf("%d%d",&n,&k);
    a[0][0]=1; b[0][0]=1;
    for(i=1;i<=n;i++)
        scanf("%f",&pi[i]);
    for(i=1;i<=n;i++)
    {
        a[i][0]=b[i][0]=1;
        for(j=1;j<=n;j++)
        {
            a[i][j]=a[i-1][j]+a[i-1][j-1]*pi[i];
            b[i][j]=b[i-1][j]+b[i-1][j-1];
        }
    }
    printf("%.6f",a[n][k]/(float)b[n][k]);
}