#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){
    //cerr<<a<<'\n';
    if(!F[a])F[a]=fact(a-1)*a; 
    return F[a];
}
 
double comb(int k,int n){
	int nk=n-k;
    long long fn=fact(n),fk=fact(k),fnk=fact(nk);
    return fn/(fnk*fk);
}

/*double comb(){
	for(int j=1;j<=n;j++){
		for(int i=1;i<=k;i++){
			
		}
	}
}*/

int main(){
	freopen ("dezastru.out","w",stdout);
	V[0]=1;
	F[0]=1;
	fin>>n>>km;
	//cerr<<fact(km)<<' '<<fact(n)<<' '<<fact(n-km)<<' '<<n-km<<'\n';
	//cerr<<comb(km,n);
	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;
}