#include <iostream>
#include <vector>
typedef long long llong;
const int MAXN = 100000 + 10;
const int INF = 1e9;
int dp[MAXN][2][2][2];
bool bl[MAXN][2][2][2];
std::vector <int> g[MAXN];
std::vector <int> dp2[MAXN][2][2][2][2];
bool state[MAXN];
int n;
int f(int node, int p, bool should, bool incoming, bool currTurned)
{
if (g[node].size() - (p != 0) == 0)
{
if ((incoming ^ should) != state[node]) return INF;
return should;
}
if (bl[node][should][incoming][currTurned])
{
return dp[node][should][incoming][currTurned];
}
bl[node][should][incoming][currTurned] = true;
if (should)
{
return dp[node][should][incoming][currTurned] = f(node, p, 0, incoming, 1) + 1;
}
dp2[node][should][incoming][currTurned][0].resize(g[node].size() - (p != 0) + 1);
dp2[node][should][incoming][currTurned][1].resize(g[node].size() - (p != 0) + 1);
for (int &i : g[node])
{
if (i == p)
{
std::swap(i, g[node].back());
break;
}
}
bool wantedParity = currTurned ^ incoming ^ state[node];
dp2[node][should][incoming][currTurned][wantedParity][g[node].size() - (p != 0)] = 0;
dp2[node][should][incoming][currTurned][!wantedParity][g[node].size() - (p != 0)] = INF;
for (int i = g[node].size() - 1 - (p != 0) ; i >= 0 ; --i)
{
for (int parity = 0 ; parity < 2 ; ++parity)
{
dp2[node][should][incoming][currTurned][parity][i] = dp2[node][should][incoming][currTurned][parity][i + 1] + f(g[node][i], node, 0, currTurned, 0);
dp2[node][should][incoming][currTurned][parity][i] = std::min(dp2[node][should][incoming][currTurned][parity][i],
dp2[node][should][incoming][currTurned][!parity][i + 1] + f(g[node][i], node, 1, currTurned, 0));
}
}
return dp[node][should][incoming][currTurned] = dp2[node][should][incoming][currTurned][0][0];
}
void solve()
{
int res = std::min(f(1, 0, 0, 0, 0), f(1, 0, 1, 0, 0));
if (res >= MAXN) std::cout << "impossible\n";
else std::cout << res << '\n';
}
void read()
{
int x, y;
std::cin >> n;
for (int i = 2 ; i <= n ; ++i)
{
std::cin >> x >> y;
g[x].push_back(y);
g[y].push_back(x);
}
for (int i = 1 ; i <= n ; ++i)
{
std::cin >> state[i];
}
}
void fastIO()
{
std::ios_base :: sync_with_stdio(0);
std::cout.tie(nullptr);
std::cin.tie(nullptr);
}
int main()
{
fastIO();
read();
solve();
return 0;
}
