#include <cstdio>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <map>
#include <vector>
using namespace std;
map<string,int>mymap;
map<string,int>::iterator it;

#define LEN 1111
bool visited[LEN];
//bool arc[LEN][LEN];
vector<int> arc[555555];
int degree[LEN];
int n,m;

bool is_v_i(int v,int i)
{
    vector<int>::iterator it=find(arc[v].begin(),arc[v].end(),i);
//    for(it=arc[v].begin();it!=arc[v].end();i++)
//    {
//        if(it)
//    }
    if(it==arc[v].end())
        return false;
    else
        return true;
}

void dfs(int v)         //深度優先遍歷
{
    visited[v]=true;
    for(int i=1;i<=n;i++)
    {
        if(!visited[i] && is_v_i(v,i))
        {
            dfs(i);
        }
    }
}

bool isConnected()        //查看遍歷後結果
{
    for(int i=1;i<=n;i++)
    {
        if(!visited[i]){return false;}
    }
    return true;
}

bool isCircuit()        //通過度數是否為偶數推斷歐拉回路
{
	int oddnum=0;
    for(int i=1;i<=n;i++)
    {
        if(degree[i]%2)
		{
			oddnum++;
			if(oddnum>2)
				return false;
		}
    }
    return true;
}



int main()
{
	#ifndef ONLINE_JUDGE
		freopen("D:/1.txt","r",stdin);
		//freopen("D:/2.txt","w",stdout);
	#endif
	int que=1;
	string s1,s2;
	while(cin>>s1>>s2)
	{
		int p,q;
		it=mymap.find(s1);
		if(it==mymap.end())
		{
			mymap[s1]=que++;
			p=que-1;
		}
		else
		{
			p=it->second;
		}
		it=mymap.find(s2);
		if(it==mymap.end())
		{
			mymap[s2]=que++;
			q=que-1;
		}
		else
		{
			q=it->second;
		}
		degree[p]++;degree[q]++;//沒方向的
        //arc[p][q]=arc[q][p]=true;//arc[p][q] ，p，q是否連通
        arc[p].push_back(q);
        arc[q].push_back(p);
	}
	n=que-1;
	dfs(1);
	if(!isConnected())
    {
        cout<<"Impossible"<<‘\n‘;
    }
    else{
        if(isCircuit())
            cout<<"Possible"<<‘\n‘;
        else
            cout<<"Impossible"<<‘\n‘;
        }
	return 0;
}
 

POJ3984 迷宮問題【水BFS】