#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define FOR(i, a, b) for (int i = a, _b = b; i <= _b; ++i)
#define FORD(i, a, b) for (int i = a, _b = b; i >= _b; --i)
#define FORLL(i, a, b) for (ll i = a, _b = b; i <= _b; ++i)
#define FORDLL(i, a, b) for (ll i = a, _b = b; i >= _b; --i)
#define all(x) x.begin(), x.end()
#define uni(x) sort(all(x)), x.erase(unique(all(x)), x.end())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define dbg(...) debug(#__VA_ARGS__, __VA_ARGS__)
template<typename T>
void __print_one(const char *&s, const T &x)
{
while (*s == ' ') ++s;
const char *p = s;
int bal = 0;
while (*s)
{
if (*s == '(') ++bal;
else if (*s == ')') --bal;
else if (*s == ',' && bal == 0) break;
++s;
}
cerr.write(p, s - p) << " = " << x;
if (*s == ',')
{
++s;
cerr << " , ";
}
}
template<typename... Args>
void debug(const char *s, Args... args)
{
cerr << "[ ";
int dummy[] = { 0 , ( __print_one(s, args) , 0 )... };
(void)dummy;
cerr << " ]\n\n";
}
template<class X>
bool maximize(X &a, X b)
{
if (a < b)
{
a = b;
return true;
}
return false;
}
template<class X>
bool minimize(X &a, X b)
{
if (a > b)
{
a = b;
return true;
}
return false;
}
// --------------------------------------------------------------------------------------------
const int maxn = 1e5 + 3;
int n, a[maxn];
ll dp[maxn][2][2];
vector<int> g[maxn];
// --------------------------------------------------------------------------------------------
void dfs(int u, int p)
{
vector<int> nodes;
for (int v : g[u])
{
if (v == p) continue;
dfs(v, u);
nodes.push_back(v);
}
if (nodes.empty())
{
dp[u][a[u]][0] = 0;
dp[u][a[u] ^ 1][1] = 1;
return;
}
FOR(j, 0, 1)
FOR(k, 0, 1)
dp[u][j][k] = k;
// **TYPE = 0 && col = a[u]
vector<ll> vec;
for (int v : g[u])
{
if (v == p) continue;
dp[u][a[u]][0] += dp[v][0][0];
vec.push_back(dp[v][0][1] - dp[v][0][0]);
}
sort(all(vec));
for (int i = 1; i <= (int)vec.size() - 1; i += 2)
{
if (vec[i] + vec[i - 1] < 0)
dp[u][a[u]][0] += vec[i] + vec[i - 1];
}
// **TYPE = 0 && col = a[u] ^ 1
vec.clear();
for (int v : g[u])
{
if (v == p) continue;
dp[u][a[u] ^ 1][0] += dp[v][0][0];
vec.push_back(dp[v][0][1] - dp[v][0][0]);
}
sort(all(vec));
if (vec.empty()) dp[u][a[u] ^ 1][0] = 1e9;
else
{
dp[u][a[u] ^ 1][0] += vec[0];
int j = 2;
while (j < (int)vec.size())
{
if (vec[j] + vec[j - 1] < 0)
dp[u][a[u] ^ 1][0] += vec[j] + vec[j - 1];
else
break;
j += 2;
}
}
// **TYPE = 1 && col = a[u]
vec.clear();
for (int v : g[u])
{
if (v == p) continue;
dp[u][a[u]][1] += dp[v][1][0];
vec.push_back(dp[v][1][1] - dp[v][1][0]);
}
sort(all(vec));
if (vec.empty()) dp[u][a[u]][1] = 1e9;
else
{
dp[u][a[u]][1] += vec[0];
// if (u == 2) dbg(dp[u][a[u]][1], dp[3][1][1], dp[3][1][0], vec[0]);
int j = 2;
while (j < (int)vec.size())
{
if (vec[j] + vec[j - 1] < 0)
dp[u][a[u]][1] += vec[j] + vec[j - 1];
else
break;
j += 2;
}
}
// **TYPE = 1 && col = a[u] ^ 1
vec.clear();
for (int v : g[u])
{
if (v == p) continue;
dp[u][a[u] ^ 1][1] += dp[v][1][0];
vec.push_back(dp[v][1][1] - dp[v][1][0]);
}
sort(all(vec));
for (int i = 1; i <= (int)vec.size() - 1; i += 2)
{
if (vec[i] + vec[i - 1] < 0)
{
dp[u][a[u] ^ 1][1] += vec[i] + vec[i - 1];
}
}
// FOR(col, 0, 1)
// FOR(type, 0, 1)
// {
// dbg(u, col, type, dp[u][col][type]);
// }
}
void solve()
{
cin >> n;
FOR(i, 2, n)
{
int u, v; cin >> u >> v;
if (u > v) swap(u, v);
g[u].push_back(v);
g[v].push_back(u);
}
FOR(i, 1, n)
cin >> a[i];
FOR(i, 1, n)
FOR(j, 0, 1)
FOR(k, 0, 1)
dp[i][j][k] = 1e9;
dfs(1, -1);
ll res = min(dp[1][0][0], dp[1][0][1]);
if (res > n)
cout << "impossible";
else
cout << res;
}
signed main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define TASK "TEST"
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
freopen(TASK".OUT", "w", stdout);
}
solve();
return 0;
}
