#include <stdio.h>
#define MAXN 25
int comb[MAXN+1][MAXN+1];
double d[MAXN+1][MAXN+1], p[MAXN+1];
int main(){
    int n, k, i, j;
    FILE *fin, *fout;
    fin=fopen("dezastru.in", "r");
    fout=fopen("dezastru.out", "w");
    fscanf(fin, "%d%d", &n, &k);
    for(i=1; i<=n; i++){
        fscanf(fin, "%lf", &p[i]);
    }
    for(i=1; i<=n; i++){
        d[i-1][0]=1;
        comb[i-1][0]=1;
        for(j=1; j<=k; j++){
            d[i][j]=d[i-1][j-1]*p[i]+d[i-1][j];
            comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
        }
    }
    fprintf(fout, "%.6lf", d[n][k]/comb[n][k]);
    fclose(fin);
    fclose(fout);
    return 0;
}