#include <bits/stdc++.h>
#define PUT(a, b) freopen(a, "r", stdin); freopen(b, "w", stdout)
using namespace std;
using ll = long long;
using ld = long double;
using uint = unsigned int;
ll n;
vector<vector<ll>> gp;
vector<ll> st;
vector<ll> bb, wb, bw, ww;
ll bbf(ll, ll);
ll wbf(ll, ll);
ll bwf(ll, ll);
ll wwf(ll, ll);
ll bbf(ll u, ll prev) {
if (bb[u] != -1) return bb[u];
vector<ll> c;
for (ll x : gp[u]) {
if (x != prev) c.push_back(x);
}
ll ret;
if (c.size() == 0) {
if (st[u] == 1) {
ret = 0;
} else {
ret = 1e9;
}
} else if (c.size() == 1) {
if (st[u] == 1) {
ret = bbf(c[0], u);
} else {
ret = wwf(c[0], u) + 1;
}
} else {
ll case1, case2;
if (st[u] == 1) {
case1 = bbf(c[0], u) + bbf(c[1], u);
case2 = wwf(c[0], u) + wwf(c[1], u) + 2;
} else {
case1 = bbf(c[0], u) + wwf(c[1], u) + 1;
case2 = wwf(c[0], u) + bbf(c[1], u) + 1;
}
ret = min(case1, case2);
}
return bb[u] = ret;
}
ll wwf(ll u, ll prev) {
if (ww[u] != -1) return ww[u];
vector<ll> c;
for (ll x : gp[u]) {
if (x != prev) c.push_back(x);
}
ll ret;
if (c.size() == 0) {
if (st[u] == 1) {
ret = 1e9;
} else {
ret = 0;
}
} else if (c.size() == 1) {
if (st[u] == 1) {
ret = bwf(c[0], u) + 1;
} else {
ret = wbf(c[0], u);
}
} else {
ll case1, case2;
if (st[u] == 1) {
case1 = bwf(c[0], u) + wbf(c[1], u) + 1;
case2 = wbf(c[0], u) + bwf(c[1], u) + 1;
} else {
case1 = bwf(c[0], u) + bwf(c[1], u);
case2 = wbf(c[0], u) + wbf(c[1], u) + 2;
}
ret = min(case1, case2);
}
return ww[u] = ret;
}
ll bwf(ll u, ll prev) {
if (bw[u] != -1) return bw[u];
vector<ll> c;
for (ll x : gp[u]) {
if (x != prev) c.push_back(x);
}
ll ret;
if (c.size() == 0) {
if (st[u] == 1) {
ret = 0;
} else {
ret = 1e9;
}
} else if (c.size() == 1) {
if (st[u] == 1) {
ret = wbf(c[0], u);
} else {
ret = bwf(c[0], u) + 1;
}
} else {
ll case1, case2;
if (st[u] == 1) {
case1 = wbf(c[0], u) + wbf(c[1], u);
case2 = bwf(c[0], u) + bwf(c[1], u) + 2;
} else {
case1 = bwf(c[0], u) + wbf(c[1], u) + 1;
case2 = wbf(c[0], u) + bwf(c[1], u) + 1;
}
ret = min(case1, case2);
}
return bw[u] = ret;
}
ll wbf(ll u, ll prev) {
if (wb[u] != -1) return wb[u];
vector<ll> c;
for (ll x : gp[u]) {
if (x != prev) c.push_back(x);
}
ll ret;
if (c.size() == 0) {
if (st[u] == 1) {
ret = 1e9;
} else {
ret = 0;
}
} else if (c.size() == 1) {
if (st[u] == 1) {
ret = wwf(c[0], u) + 1;
} else {
ret = bbf(c[0], u);
}
} else {
ll case1, case2;
if (st[u] == 1) {
case1 = bbf(c[0], u) + wwf(c[1], u) + 1;
case2 = wwf(c[0], u) + bbf(c[1], u) + 1;
} else {
case1 = wwf(c[0], u) + wwf(c[1], u) + 2;
case2 = bbf(c[0], u) + bbf(c[1], u);
}
ret = min(case1, case2);
}
return wb[u] = ret;
}
void solve() {
cin >> n;
gp = vector<vector<ll>>(n);
st = vector<ll>(n);
bb = wb = bw = ww = vector<ll>(n, -1);
for (ll i = 1; i < n; i++) {
ll u, v;
cin >> u >> v;
u--, v--;
gp[u].push_back(v);
gp[v].push_back(u);
}
for (ll& x : st) cin >> x, x = !x;
ll ans = 1e9;
for (ll u = 0; u < n; u++) {
if (gp[u].size() == 1) {
ans = min(wwf(u, -1) + 1, bbf(u, -1));
break;
}
}
if (ans >= 1e9) cout << "impossible" << endl;
else cout << ans << endl;
}
int main() {
//cin.tie(NULL); ios_base::sync_with_stdio(false);
//int t; cin >> t; while (t--)
solve();
}
