#include <cstdio>

using namespace std;
double P[26],DP[26][26];
int Comb[26][26];
int N,K;

void read()
{
    scanf("%d%d",&N,&K);
    for(int i = 1; i <= N; ++i)
        scanf("%lf",P+i);
}

void solve()
{
    Comb[0][0] = 1;
    DP[0][0] = 1;
    for(int i = 1; i <= N; ++i)
    {
        DP[i][0] = 1;
        Comb[i][0] = 1;
        for(int j = 1; j <= K; ++j)
        {
            DP[i][j] = DP[i-1][j] + P[i]*DP[i-1][j-1];
            Comb[i][j] = Comb[i-1][j] + Comb[i-1][j-1];
        }
    }
    printf("%.6lf",DP[N][K]/Comb[N][K]);
}

int main()
{
    freopen ( "dezastru.in", "r", stdin );
    freopen ( "dezastru.out", "w", stdout );

    read();
    solve();

    return 0;
}