#include <stdio.h>
#define maxn 30

int n,k,i,j,c[maxn][maxn];
float v[maxn],d[maxn][maxn];

int main()
{
    FILE *f1,*f2;
    f1=fopen("dezastru.in","r");
    f2=fopen("dezastru.out","w");

    fscanf(f1,"%d%d",&n,&k);

    for(i=1;i<=n;i++)
        fscanf(f1,"%f",&v[i]);

    c[1][1]=c[1][0]=d[1][0]=1;
    d[1][1]=v[1];

    for(i=2;i<=n;i++)
    {
        c[i][0]=d[i][0]=1;
        for(j=1;j<=k;j++)
        {
            c[i][j]=c[i-1][j-1] + c[i-1][j];
            d[i][j]=d[i-1][j-1]*v[i] + d[i-1][j];
        }
    }

    fprintf(f2,"%f",d[n][k]/c[n][k]);

    return 0;
}