#include <bits/stdc++.h>
 
using namespace std;
 
#define NMAX 26
 
ifstream fin("dezastru.in");
 
double s;
int n,km,S[NMAX],F[NMAX];
double P[NMAX],V[NMAX],PD[NMAX][NMAX];
 
void dp(){
    for(int i=0;i<=n;i++) PD[i][0]=1;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=km;j++){
            PD[i][j]=PD[i-1][j]+PD[i-1][j-1]*P[i];
        }
    }
}
 
long long fact(int a){
    if(!F[a])F[a]=fact(a-1)*a; 
    return F[a];
}
 
long long comb(int k,int n){
    long long fn=fact(n),fk=fact(k),fnk=fact(n-k);
    return fn/(fk*fnk);
}
 
int main(){
    freopen ("dezastru.out","w",stdout);
    V[0]=1;
    F[1]=1;
    fin>>n>>km;
    for(int i=1;i<=n;i++){ 
        fin>>P[i];
    }
    dp();
    s = PD[n][km]/comb(km,n);
    printf("%.6f",s);
    //cerr<<clock()*1000.0/CLOCKS_PER_SEC<<'\n';
    return 0;
}