#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll MAXN = 1e5 + 5;
ll n, ans = 0, val[MAXN];
bool flag = true;
ll cek(ll a, ll b, ll c) {
if (a == 1) {
if (b == 1) {
if (c == 1) {
return 2;
} else {
return 4;
}
} else {
if (c == 1) {
return 5;
} else {
return 3;
}
}
} else {
if (b == 1) {
if (c == 1) {
return 1;
} else {
return 7;
}
} else {
if (c == 1) {
return 6;
} else {
return 0;
}
}
}
}
void update(ll state) {
bitset<3> bs = state;
ans += bs.count();
}
int main(){
cin >> n;
for (int i = 0; i < n-1; ++i)
{
ll tmp;
cin >> tmp >> tmp;
}
for (int i = 1; i <= n; ++i)
{
cin >> val[i];
}
for (int i = 1; i <= n; ++i)
{
if (!flag)
{
break ;
}
if (n-i+1 < 3)
{
if (n-i+1 == 2) {
if (val[i] != val[i+1])
{
flag = false;
}
break ;
} else {
if (val[i] == 1)
{
flag = false;
}
break ;
}
} else {
ll state = cek(val[i], val[i+1], val[i+2]);
bitset<3> bs = state;
update(state);
for (int j = 0; j < 3; ++j)
{
// Sebelum
if (j == 2 && bs[j] && i != 1)
{
if (val[i-1] == 0)
{
flag = false;
break ;
}
}
// Sesudah
if (j == 0 && bs[j] && i != n)
{
val[i+1] = val[i+1] ^ 1ll;
}
}
}
}
if (flag) {
cout << ans << "\n";
} else {
cout << "impossible\n";
}
return 0;
}
