#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
const int N=33000;
int n,m,x,i,j,k,pos[N],a[N],b[N],at[N],st[N],en[N],f[N];double ans;
struct comp{double r,i;comp(double _r=0,double _i=0){r=_r;i=_i;}comp operator+(const comp&x){return comp(r+x.r,i+x.i);}comp operator-(const comp&x){return comp(r-x.r,i-x.i);}comp operator*(const comp&x){return comp(r*x.r-i*x.i,r*x.i+i*x.r);}comp conj(){return comp(r,-i);}
}A[N],B[N];
const double pi=acos(-1.0);
void FFT(comp a[],int n,int t){for(int i=1;i<n;i++)if(i<pos[i])swap(a[i],a[pos[i]]);for(int d=0;(1<<d)<n;d++){int m=1<<d,m2=m<<1;double o=pi*2/m2*t;comp _w(cos(o),sin(o));for(int i=0;i<n;i+=m2){comp w(1,0);for(int j=0;j<m;j++){comp&A=a[i+j+m],&B=a[i+j],t=w*A;A=B-t;B=B+t;w=w*_w;}}}if(t==-1)for(int i=0;i<n;i++)a[i].r/=n;
}
inline int lower(int x){int l=1,r=m,mid,t;while(l<=r)if(b[mid=(l+r)>>1]<=x)l=(t=mid)+1;else r=mid-1;return t;
}
int main(){scanf("%d",&n);for(i=1;i<=n;i++)scanf("%d",&a[i]),b[i]=a[i];sort(b+1,b+n+1);for(i=1;i<=n;i++)if(i==1||b[i]!=b[i-1])b[++m]=b[i];for(i=1;i<=n;i++)a[i]=lower(a[i]);for(i=1;i<=n;i++)at[i]=(i-1)/1500+1;for(i=1;i<=n;i++)en[at[i]]=i;for(i=n;i;i--)st[at[i]]=i;for(k=1;k<=m;k<<=1);k<<=1;j=__builtin_ctz(k)-1;for(i=0;i<k;i++)pos[i]=pos[i>>1]>>1|((i&1)<<j);for(x=1;x<=at[n];x++){for(i=st[x];i<=en[x];i++)for(j=i+1;j<=en[x];j++)if(a[i]>a[j])f[a[i]+m-a[j]]++;if(x==1)continue;for(i=0;i<k;i++)A[i]=comp();for(i=st[x]-1;i;i--)A[a[i]].r+=1.0;for(i=st[x];i<=en[x];i++)A[m-a[i]].i+=1.0;FFT(A,k,1);for(i=0;i<k;i++){j=(k-i)&(k-1);B[i]=B[i]+A[i]*A[i]-(A[j]*A[j]).conj();}}for(i=0;i<k;i++)B[i]=B[i]*comp(0,-0.25);FFT(B,k,-1);for(i=1;i<m;i++)ans+=2.0/(i+1)*(f[i+m]+int(B[i+m].r+0.5));return printf("%.6f",ans),0;
}