#include<cstdio>
int n,k,i,j;
double v[30],c[30][30],d[30][30];
FILE *f,*g;
int main(){
    f=fopen("dezastru.in","r");
    g=fopen("dezastru.out","w");
    fscanf(f,"%d%d",&n,&k);
    for(i=1;i<=n;i++){
        fscanf(f,"%lf",&v[i]);
    }
    c[0][0]=d[0][0]=1;
    for(i=1;i<=n;i++){
        c[i][0]=1;
        d[i][0]=1;
        for(j=1;j<=k;j++){
            c[i][j] = c[i-1][j] + c[i-1][j-1];
            d[i][j] = d[i-1][j-1] * v[i] + d[i-1][j];
        }
    }
    fprintf(g,"%.6lf",d[n][k]/c[n][k]);

    fclose(f);
    fclose(g);
    return 0;
}