#include <cstdio>
#define nmax 26

FILE *fin = fopen("dezastru.in", "r"), *fout = fopen("dezastru.out", "w");

int n, k;
float p[nmax];


float dinamic()
{
    int i,j;
    float r;

    for(i = 1; i <= n ; i++)
        for(j = i+1 ; j <= n ;j++)
            r += p[i] * p[j];

    r *= k;
    r /= (n*k);

    return r;
}


int main()
{
    fscanf(fin, "%d %d", &n, &k);

    for( int i = 1; i <= n; i++)
        fscanf(fin, "%f", &p[i]);

    fprintf(fout,"%.6f", dinamic());


    return 0;
}