#include <fstream>

using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double comb[105][105],v[105],s;
int n,m,i,j;
int main()
{
    f>>n>>m;
    for(i=1;i<=n;++i)
        f>>v[i];
    for(i=0;i<=n;++i)
        comb[i][0]=1;
    for(i=1;i<=n;++i)
        for(j=1;j<=m;++j)
            comb[i][j]=comb[i-1][j]+v[i]*comb[i-1][j-1];
    s=comb[n][m];
    for(i=1;i<=m;++i)
    {
        s/=(n-m+i);
        s=s*i;
    }
    g<<s;
    return 0;
}