#include <stdio.h>

double v[26], d[26][26], c[26][26];
int main(){
    FILE*fi,*fo;
    fi=fopen("dezastru.in","r");
    fo=fopen("dezastru.out","w");
    int n, k;
    fscanf(fi,"%d%d", &n, &k);
    for(int i=1;i<=n;i++) fscanf(fi,"%lf", &v[i]);
    for(int i=0;i<=n;i++) d[i][0]=c[i][0]=1.0;
    for(int i=1;i<=n;i++)
        for(int j=1;j<=k;j++){
            d[i][j]=d[i-1][j]+d[i-1][j-1]*v[i];
            c[i][j]=c[i-1][j]+c[i-1][j-1];
        }
    fprintf(fo,"%.6lf", d[n][k]/c[n][k]);
    return 0;
}