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#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<set>
#include<bitset>
#include<complex>
#include<assert.h>
#define inc(i,l,r) for(int i=l;i<=r;i++)
#define dec(i,l,r) for(int i=l;i>=r;i--)
#define link(x) for(edge *j=h[x];j;j=j->next)
#define mem(a) memset(a,0,sizeof(a))
#define ll long long
#define eps 1e-8
#define succ(x) (1<<x)
#define lowbit(x) (x&(-x))
#define sqr(x) ((x)*(x))
#define mid (x+y)/2
#define NM 50005
#define nm 150005
using namespace std;
const double pi=acos(-1);
const ll inf=2013265921;
ll read(){
ll x=0,f=1;char ch=getchar();
while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
while(isdigit(ch))x=x*10+ch-'0',ch=getchar();
return f*x;
}
struct edge{int t;edge*next;}e[nm],*h[NM],*o=e;
void add(int x,int y){o->t=y;o->next=h[x];h[x]=o++;}
int n,m,size[NM],tot,smin,root,_x,_y,rev[nm];
bool v[NM],_v[nm];
ll a[nm],b[nm],c[nm],ans;
ll qpow(ll x,ll t){return t?qpow(sqr(x)%inf,t>>1)*(t&1?x:1ll)%inf:1ll;}
void dfs2(int x,int f){size[x]=1;link(x)if(!v[j->t]&&j->t!=f)dfs2(j->t,x),size[x]+=size[j->t];}
void getroot(int x,int f){
int s=tot-size[x];
link(x)if(!v[j->t]&&j->t!=f)getroot(j->t,x),s=max(s,size[j->t]);
if(s<smin)smin=s,root=x;
}
void fft(ll*a,int f){
inc(i,0,n-1)if(i<rev[i])swap(a[i],a[rev[i]]);
for(int k=1;k<n;k<<=1){
ll t=qpow(31,(inf-1)/k/2);if(f==-1)t=qpow(t,inf-2);
for(int i=0;i<n;i+=k<<1){
ll w=1;
for(int j=0;j<k;j++,w=w*t%inf){
ll x=a[i+j],y=w*a[i+j+k]%inf;
a[i+j]=(x+y)%inf;a[i+j+k]=(x-y+inf)%inf;
}
}
}
}
void plu(ll*a,ll*b){
int bit=0;
n=n+m-1;
while(succ(bit)<n)bit++;n=succ(bit);
inc(i,m,n)b[i]=0;
ll invn=qpow(n,inf-2);
inc(i,1,n-1)rev[i]=(rev[i>>1]>>1)|((i&1)<<(bit-1));
fft(a,1);fft(b,1);inc(i,0,n-1)a[i]=a[i]*b[i]%inf;
fft(a,-1);inc(i,0,n-1)a[i]=a[i]*invn%inf;
}
void dfs(int x,int f,int t){
a[t]++;tot=max(tot,t);
link(x)if(!v[j->t]&&j->t!=f)dfs(j->t,x,t+1);
}
void div(int x){
dfs2(x,0);
tot=size[x];smin=inf;
getroot(x,0);tot=0;
dfs(root,0,0);
v[root]++;
inc(i,0,tot)c[i]=a[i],a[i]=0;
inc(i,2,tot)if(!_v[i])ans+=c[i];
m=tot+1;
link(root)if(!v[j->t]){
tot=0;
dfs(j->t,root,1);
inc(i,0,m-1)b[i]=c[i]-a[i];
n=tot+1;
plu(a,b);
inc(i,0,n-1){if(!_v[i])ans+=a[i];a[i]=0;}
}
link(root)if(!v[j->t])div(j->t);
}
int main(){
n=1e5;_v[1]++;_v[0]++;
inc(i,2,n)if(!_v[i])for(int j=i<<1;j<=n;j+=i)_v[j]++;
n=read();inc(i,2,n){_x=read();_y=read();add(_x,_y);add(_y,_x);}
_x=n;
div(1);
return 0*printf("%.7lf\n",1.0*ans/_x/(_x-1));
}
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