#include <bits/stdc++.h>
using namespace std;
void setup()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}
int n, a, b, f[100000][2][2], res;
bool state[100000];
vector<int> g[100000];
inline void DFS(int node, int par)
{
int c[2][2] = {{0, 0}, {(int)1e9, (int)1e9}}, d[2][2];
for (auto &i : g[node])
{
if (i != par)
{
DFS(i, node);
for (int j = 0; j < 2; ++j)
{
for (int k = 0; k < 2; ++k)
{
d[j][k] = min(c[j][k] + f[i][0][k], c[1 - j][k] + f[i][1][k]);
}
}
for (int j = 0; j < 2; ++j)
{
for (int k = 0; k < 2; ++k)
{
c[j][k] = min((int)1e9, d[j][k]);
}
}
}
}
f[node][0][0] = c[state[node]][0];
f[node][0][1] = c[1 - state[node]][0];
f[node][1][0] = min((int)1e9, c[1 - state[node]][1] + 1);
f[node][1][1] = min((int)1e9, c[state[node]][1] + 1);
}
int main()
{
setup();
cin >> n;
for (int i = 0; i < n - 1; ++i)
{
cin >> a >> b;
g[a - 1].push_back(b - 1);
g[b - 1].push_back(a - 1);
}
for (int i = 0; i < n; ++i)
{
cin >> state[i];
}
DFS(0, 0);
res = min(f[0][0][0], f[0][1][0]);
if (res > 1e6)
{
cout << "Impossible";
return 0;
}
cout << res;
return 0;
}
