#include <bits/stdc++.h>
///N=25
///K=25
using namespace std;
int n, k, i, j;
float val[26], sol[26][26], fact[26];
void read();
void solve();
void write();
void bkt(int i);
int main()
{
    read();
    solve();
    write();
    return 0;
}
void read(){
    freopen("dezastru.in", "r", stdin);
    scanf("%d%d", &n, &k);
    for(i=1; i<=n; ++i) scanf("%f", &val[i]);
    fclose(stdin);
}
void solve(){
    fact[0]=1.0;
    sol[0][0]=1.0;
    for(i=1; i<=n; ++i){
        sol[i][0]=1.0;
        for(j=1; j<=i; ++j) sol[i][j]=sol[i-1][j]+(sol[i-1][j-1]*val[i]);
        fact[i]=fact[i-1]*i;
    }
}
void write(){
    freopen("dezastru.out", "w", stdout);
    printf("%f", sol[n][k]*fact[k]*fact[n-k]/fact[n]);
    fclose(stdout);
}