#include <cstdio>

using namespace std;
float v[26][26],pi[26];
long long comb[26][26];
int main()
{   int n,i,j,k;
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    v[0][0]=1;comb[0][0]=1;
    for(i=1;i<=n;i++) scanf("%f",&pi[i]);
    for(i=1;i<=n;i++)
    {
        v[i][0]=comb[i][0]=1;
        for(j=1;j<=n;j++) v[i][j]=v[i-1][j]+v[i-1][j-1]*pi[i],comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
    }
    printf("%.6f",v[n][k]/(float)comb[n][k]);
}