#include <bits/stdc++.h>
using namespace std;
#define pb push_back
typedef pair<int, int> pii;
#define fi first
#define se second
const int MAXN = 100005;
int n, po[MAXN], pc = 0;
bitset<MAXN> a;
basic_string<int> al[MAXN];
void dfs(int x){
po[x] = pc++;
for(auto nx: al[x]){
if(po[nx] || nx == 0) continue;
dfs(nx);
}
}
typedef array<int, 2> aii;
aii memo[MAXN][2];
aii dp(int x, bool tog){
aii& mem = memo[x][tog];
if(mem[0] != -1){
return mem;
}
int mc=0, mt=MAXN;
bool mct=0;
for(auto nx: al[x]){
if(po[nx] < po[x]) continue;
int ct0 = dp(nx, 0)[tog], ct1 = dp(nx, 1)[tog];
if(ct0 < ct1){
mc += ct0;
}
else{
mct = !mct;
mc += ct1;
}
mt = min(mt, abs(ct0 - ct1));
}
bool cs = mct ^ tog ^ a[x];
mem = {mc + tog, mc + tog + mt};
if(cs){
swap(mem[0], mem[1]);
}
return mem;
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
cin >> n;
for(int i=0, a, b; i<n-1; i++){
cin >> a >> b;
a--; b--;
al[a].pb(b);
al[b].pb(a);
}
for(int i=0; i<n; i++){
bool x;
cin >> x;
a[i] = x;
}
dfs(0);
memset(memo, -1, sizeof(memo));
int ans = min(dp(0, 0)[0], dp(0, 1)[0]);
if(ans >= MAXN){
cout << "impossible";
}
else{
cout << ans;
}
return 0;
}
