#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
#define maxn 30
double DP[maxn][maxn], P[maxn];
int main(){
    freopen ("dezastru.in", "r", stdin);
    freopen ("dezastru.out", "w", stdout);
    int N, K;
    scanf ("%d %d", &N, &K);
    for(int i=1;i<=N;++i) scanf ("%lf", &P[i]);
    for(int i=0;i<=N;++i) DP[i][0]=1;
    for(int i=1;i<=N;++i)
        for(int j=1;j<=K&&j<=i;++j) DP[i][j]=DP[i-1][j]+DP[i-1][j-1]*P[i];
    for(int i=1;i<=N-K;++i) DP[N][K]*=i;
    for(int i=K+1;i<=N;++i) DP[N][K]/=i;
    printf("%.7lf", DP[N][K]);
    return 0;
}