#include<fstream>
using namespace std;
ifstream cin("dezastru.in");
ofstream cout("dezastru.out");

int n,m;
double v[105];
double comb[105][105];
double s;

int main(){

    cin>>n>>m;
    for(int i=1;i<=n;i++)
        cin>>v[i];

    for(int i=0;i<=n;i++)
        comb[i][0]=1;
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++)
            comb[i][j]=comb[i-1][j]+v[i]*comb[i-1][j-1];

    s=comb[n][m];

    for(int i=1;i<=m;i++){

        s/=n-m+i;
        s*=i;

    }

    cout<<s;

}