求调，RE最后一个点

#include <bits/stdc++.h>
using namespace std;

const int N = 2.5e5 + 5;

int n;
map<string, int> mp;
vector<int> g[N];
bool vis[N];

void dfs(int u) {
    vis[u] = 1;
    for (int v : g[u]) {
        if (!vis[v]) {
            dfs(v);
        }
    }
    return ;
}

bool euler() {
    memset(vis, 0, sizeof vis);
    dfs(1);
    int num = 0;
    for (int i = 1; i <= n; i++) {
        if (g[i].size() & 1) num++;
        if (!vis[i]) return 0;
    }
    return num == 0 || num == 2;
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    
    string a, b;
    while (cin >> a >> b) {
        if (!mp[a]) mp[a] = ++n;
        if (!mp[b]) mp[b] = ++n;
        g[mp[a]].push_back(mp[b]);
        g[mp[b]].push_back(mp[a]);
    }
    cout << (euler() ? "Possible" : "Impossible") << "\n";
    return 0;
}