#include<stdio.h>
int nrComb[26][26];
float v[25],ans[26][26];
int main()
{
    FILE *fin,*fout;
    fin=fopen("dezastru.in","r");
    fout=fopen("dezastru.out","w");
    int n,k;
    fscanf(fin,"%d%d",&n,&k);
    int i;
    for(i=0; i<n; i++)
        fscanf(fin,"%f",&v[i]);
    int j;
    nrComb[0][0]=ans[0][0]=1;
    for(i=1; i<=n; i++)
    {
        nrComb[i][0]=ans[i][0]=1;
        for(j=1; j<=n; j++)
        {
            nrComb[i][j]=nrComb[i-1][j]+nrComb[i-1][j-1];
            ans[i][j]=ans[i-1][j]+ans[i-1][j-1]*v[i-1];
        }
    }
    fprintf(fout,"%.6f",ans[n][k]/nrComb[n][k]);
    return 0;
}