#include<bits/stdc++.h>
using namespace std;
const int N=2e5+10;
vector<pair<int,int>>G[N];
int id,cnt,dfn[N],low[N],cut[N];
int root;int st,ed;
void tarjan(int x,int in_id)
{
    dfn[x]=low[x]=++id;
    int child=0;
    for(auto i:G[x])if(i.second!=in_id)
	{
        int y=i.first,id=i.second;
        if(dfn[y]==0)
		{
            tarjan(y,id);
            low[x]=min(low[x],low[y]);
            if(dfn[x]<=low[y])
			{
                child++;
                if(x!=root||child>=2){
                    if(dfn[y]<=dfn[ed])  cut[x]=1;
                }
            }
        }
        else low[x]=min(dfn[y],low[x]);
    }
}
int main()
{
    int n;scanf("%d",&n);
    for(int i=1,x,y;scanf("%d%d",&x,&y);i++)
    {
        if(x==0 && y==0)break;
        G[x].push_back({y,i});
        G[y].push_back({x,i});
    }

    scanf("%d%d",&st,&ed);
    id=cnt=0;memset(dfn,0,sizeof(dfn));memset(low,0,sizeof(low)); 
    memset(cut,0,sizeof(cut)); 
	root=st,tarjan(st,0);
	
    for(int i=1;i<=n;i++)if(cut[i]&&i!=st&&i!=ed){printf("%d\n",i);return 0;}
    puts("No solution");
    return 0;
}