#include <iostream>
#include <vector>
#include <bitset>
#include <algorithm>
using namespace std;
using ll = long long;
const int MAXN = 1e5 + 123;
vector<int> edg[MAXN];
ll dp[MAXN][4];
bitset<MAXN> bs;
ll get_mxsm(vector<ll>& v, size_t stid) {
ll ans = 0;
for (size_t i = stid; i + 1 < v.size() && v[i] + v[i + 1] < 0; i += 2)
ans += v[i] + v[i + 1];
return ans;
}
void dfs(int v, int rt) {
int p = bs[v];
if (rt != -1 && edg[v].size() == 1) {
dp[v][p] = 0;
dp[v][(p ^ 1) + 2] = 1;
dp[v][p ^ 1] = MAXN;
dp[v][p + 2] = MAXN;
return;
}
ll sum0 = 0;
ll sum1 = 0;
vector<ll> v0;
vector<ll> v1;
for (auto u : edg[v]) {
if (u == rt)
continue;
dfs(u, v);
sum0 += dp[u][0];
sum1 += dp[u][1];
v0.push_back(dp[u][2] - dp[u][0]);
v1.push_back(dp[u][3] - dp[u][1]);
}
sort(v0.begin(), v0.end());
sort(v1.begin(), v1.end());
// bs[v] = 0
dp[v][0] = sum0 + get_mxsm(v0, 0);
dp[v][1] = sum0 + v0[0] + get_mxsm(v0, 1);
dp[v][2] = 1 + sum1 + v1[0] + get_mxsm(v1, 1);
dp[v][3] = 1 + sum1 + get_mxsm(v1, 0);
if (bs[v]) {
swap(dp[v][0], dp[v][1]);
swap(dp[v][2], dp[v][3]);
}
}
signed main() {
int n;
cin >> n;
for (int i = 1; i < n; ++i) {
int u, v;
cin >> u >> v;
--u, --v;
edg[u].push_back(v);
edg[v].push_back(u);
}
for (int i = 0; i < n; ++i) {
char c;
cin >> c;
bs[i] = c == '1';
}
dfs(0, -1);
ll ans = min(dp[0][0], dp[0][2]);
if (ans > n)
cout << "impossible\n";
else
cout << ans << '\n';
//for (int i = 0; i < n; ++i) {
//cout << dp[i][0] << ' ' << dp[i][1] << ' ' << dp[i][2] << ' ' << dp[i][3] << endl;
//}
}
