CodeForces - 626D

#include <iostream>
#include <cstdio>
#include <queue>
#include <algorithm>
#include <cmath>
#include <cstring>
#define inf 2147483647
#define N 1000010
#define p(a) putchar(a)
#define For(i,a,b) for(int i=a;i<=b;++i)

using namespace std;
int n;
double a[N],sum[N],cnt[N],ans,t;
void in(int &x){
    int y=1;char c=getchar();x=0;
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)y=-1;c=getchar();}
    while(c<=‘9‘&&c>=‘0‘){ x=(x<<1)+(x<<3)+c-‘0‘;c=getchar();}
    x*=y;
}
void o(int x){
    if(x<0){p(‘-‘);x=-x;}
    if(x>9)o(x/10);
    p(x%10+‘0‘);
}

signed main(){
    in(n);
    For(i,1,n) cin>>a[i];
    sort(a+1,a+n+1);
    For(i,2,n)
        For(j,1,i-1)
            cnt[int(a[i]-a[j])]=cnt[int(a[i]-a[j])]+1.0;
    For(i,1,5000) sum[i]=sum[i-1]+cnt[i];
    t=n*(n-1)/2;
    For(i,1,5000)
        For(j,1,5000){
            if(i+j<5000){
                ans+=cnt[i]/t*cnt[j]/t*(sum[5000]-sum[i+j])/t;
            }
        }
    printf("%.10f\n",ans);
    return 0;
}