#include<iostream>
#include<vector>
#include<queue>
using namespace std;
using pi = pair<int, int>;
using pii = pair<pi, pi>;
int main() {
int n; cin >> n;
vector<vector<int>> g(n);
for (int i = 0; i < n - 1; i++)
{
int a, b; cin >> a >> b; a--; b--;
g[b].push_back(a); g[a].push_back(b);
}
vector<int> v(n);
for (int i = 0; i < n; i++)
{
cin >> v[i];
}
vector<int> p(n), ch(n);
queue<int> q; q.push(0);
while (!q.empty())
{
int x = q.front(); q.pop();
for (int y : g[x])
{
if (p[x] != y)
{
p[y] = x;
q.push(y);
ch[x]++;
}
}
}
for (int i = 0; i < n; i++)
{
if (!ch[i])
{
q.push(i);
}
}
vector<pii> dp(n);
//vector<pi> dp2(n);
int inf = n + 1;
while (!q.empty())
{
int x = q.front(); q.pop();
pi p1 = { 0, inf };
pi p2 = { inf, 1 };
if (v[x])
{
p1 = { inf, 0 };
p2 = { 1, inf };
}
for (int y : g[x])
{
if (p[x] != y)
{
pi pp;
pp.first = min(p1.first + dp[y].first.first,
p1.second + dp[y].second.first);
pp.second = min(p1.second + dp[y].first.first,
p1.first + dp[y].second.first);
p1 = pp;
pp.first = min(p2.first + dp[y].first.second,
p2.second + dp[y].second.second);
pp.second = min(p2.second + dp[y].first.second,
p2.first + dp[y].second.second);
p2 = pp;
}
}
dp[x] = { p1, p2 };
ch[p[x]]--;
if (!ch[p[x]])
{
q.push(p[x]);
}
}
int r = min(dp[0].first.first, dp[0].second.first);
if (r >= inf)
{
cout << "IMPOSSIBLE" << endl;
return 0;
}
cout << r << endl;
}
