#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<vector>
using namespace std;
const int SIZEN=100010;
int N;
vector<int> c[SIZEN];
double x[SIZEN]={0},y[SIZEN]={0};
double ans=0;
double sz[SIZEN]={0};
double xsum[SIZEN]={0};
void DFS(int u,int fa){
	sz[u]=1;
	for(int i=0;i<c[u].size();i++){
		int v=c[u][i];
		if(v!=fa){
			DFS(v,u);
			sz[u]+=sz[v];
			xsum[u]+=xsum[v];
			ans+=y[u]*xsum[v]*sz[v];
		}
	}
	xsum[u]+=x[u];
	ans+=y[u]*(1-xsum[u])*(N-sz[u]);
}
void re_calc(double x[]){
	double s=0;
	for(int i=1;i<=N;i++) s+=x[i];
	for(int i=1;i<=N;i++) x[i]/=s;
}
void read(void){
	scanf("%d",&N);
	int a,b;
	for(int i=1;i<N;i++){
		scanf("%d%d",&a,&b);
		c[a].push_back(b);
		c[b].push_back(a);
	}
	for(int i=1;i<=N;i++) scanf("%lf%lf",&x[i],&y[i]);
	re_calc(x);
	re_calc(y);
}
int main(){
	freopen("cf123e_maze.in","r",stdin);
	freopen("cf123e_maze.out","w",stdout);
	read();
	DFS(1,0);
	printf("%.15lf\n",ans);
	return 0;
}