#include <cstdio>
using namespace std;

double s, v[30];
int n, k, var;

inline void backtracking( int niv, int last, double prob ){

    int i;

    if( niv - 1 ==  k ){
        s += prob;
        var++;
        return ;
    }
    else{

        for( i = last + 1; i <= n - k + niv; ++i ){
            backtracking( niv + 1, i, prob * v[i] );
        }

    }

}

int main()
{


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

    int  i, j;

    scanf("%d%d",&n,&k);

    for( i = 1; i <= n; ++i ){
        scanf("%lf",&v[i]);
    }


    backtracking( 1, 0, 1 );

    printf("%.6lf",s/(var*1.0));

    return 0;
}