#include<stdio.h>
int n,k,i,j,num,c[26];
float s,pr,p[26];
int main()
{
freopen("dezastru.in","r",stdin);
freopen("dezastru.out","w",stdout);

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

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

for(i=1;i<n;++i)
   for(j=i+1;j<=n;++j)
      {
      pr=p[i]*p[j];
      s+=pr;
      ++num;
      }
s=s/num;
printf("%.6f",s);
return 0;
}