#define NDEBUG
#ifdef NDEBUG
#define dbg(TXTMSG) if constexpr (false) cerr
#define dbgv(VARN) ((void)0)
#define dbg_for if constexpr (false) for
#else
#pragma GCC optimize("trapv")
#define _GLIBCXX_DEBUG 1
#define _GLIBCXX_DEBUG_PEDANTIC 1
#define dbg_for for
#define dbg(TXTMSG) cerr << "\n" << (TXTMSG)
#define dbgv(VARN) cerr << "\n" << (#VARN) << " = "<< (VARN) << ", line: " << __LINE__ << "\n"
#endif
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll,ll>;
constexpr ll INFTY = 1e10;
#define var const auto&
/*
node_black_untoggled, node_black_toggled;
node_white_untoggled, node_white_toggled;
*/
using dp_state = array<array<ll,2>,2> ;
int main() {
ll N;
cin >> N;
vector<vector<ll>> cam_connexion(N,vector<ll>());
for (int i = 0; i < (N-1); ++i)
{
ll ai,bi;
cin >> ai >> bi;
ai--;bi--; //c
cam_connexion[ai].push_back(bi);
cam_connexion[bi].push_back(ai);
}
//0-off 1-on
vector<ll> initial_state(N,-1);
for (int i = 0; i < N; ++i)
{
cin >> initial_state[i];
}
// >=INFTY signifies impossiblity
#define UN_PROCESSED dp_state{array<ll,2>{-1,-1},array<ll,2>{-1,-1}}
#define BEING_PROCESSED dp_state{array<ll,2>{-2,-2},array<ll,2>{-2,-2}}
vector<dp_state> subtree_bests(N,UN_PROCESSED);
//cam-0 taken as root
stack<ll> dfs({0});
while (!dfs.empty()) {
ll curr_node = dfs.top();
if (subtree_bests[curr_node]==UN_PROCESSED) {
for (var relative : cam_connexion[curr_node])
{
if (subtree_bests[relative]!=BEING_PROCESSED) {
assert(subtree_bests[relative]==UN_PROCESSED);
dfs.push(relative);
}
}
subtree_bests[curr_node]=BEING_PROCESSED;
}
else if (subtree_bests[curr_node]==BEING_PROCESSED) {
if (cam_connexion[curr_node].size()==1 && curr_node!=0) {
if (initial_state[curr_node]==0) {
subtree_bests[curr_node]=dp_state{array<ll,2>{0,INFTY},array<ll,2>{INFTY,1}};
}
else {
subtree_bests[curr_node]=dp_state{array<ll,2>{INFTY,1},array<ll,2>{0,INFTY}};
}
dfs.pop();
continue;
}
for (ll toggled = 0; toggled <=1; toggled++)
{
ll res_toggle_of_best = 0;
ll bestcost = toggled;
ll leastdiff = INFTY;
for (var relative : cam_connexion[curr_node])
{
if (subtree_bests[relative]!=BEING_PROCESSED) {
assert(subtree_bests[relative]!=UN_PROCESSED);
if (subtree_bests[relative][toggled][0]==INFTY && subtree_bests[relative][toggled][1]==INFTY) {
subtree_bests[curr_node][0][toggled] = INFTY;
subtree_bests[curr_node][1][toggled] = INFTY;
goto caseToggled;
}
bestcost+=min(subtree_bests[relative][toggled][0],subtree_bests[relative][toggled][1]);
res_toggle_of_best ^= subtree_bests[relative][toggled][1]<subtree_bests[relative][toggled][0];
if (subtree_bests[relative][toggled][0]!= INFTY && subtree_bests[relative][toggled][1]!=INFTY) {
leastdiff = min(leastdiff,abs(subtree_bests[relative][toggled][1]-subtree_bests[relative][toggled][0]));
}
}
}
dbgv(curr_node);dbgv(toggled);dbgv(res_toggle_of_best);dbgv(bestcost);dbgv(leastdiff);
dbgv((res_toggle_of_best^initial_state[curr_node]^toggled));
subtree_bests[curr_node][res_toggle_of_best^initial_state[curr_node]^toggled][toggled] = bestcost;
subtree_bests[curr_node][res_toggle_of_best^initial_state[curr_node]^1^toggled][toggled] = min(INFTY,bestcost+leastdiff);
caseToggled:;
}
dfs.pop();
}
else throw exception();
}
dbg_for (auto [a,b] : subtree_bests)
{
dbg("next:")<< a[0] << " " << a[1] << "|" << b[0] << " " << b[1];
}
ll ans = min(subtree_bests[0][0][0],subtree_bests[0][0][1]);
if (ans==INFTY){
cout << "impossible";
}
else {
cout << ans;
}
}
