#include <bits/stdc++.h>
using namespace std;
#define MAXN 200001
#define int long long
int n;
vector<pair<string,string>> edges;
vector<string> kompresija;
bool loved[MAXN];
vector<int> adj[MAXN];
bool pos[MAXN];
int siz=0;
int dist[MAXN];
vector<int> nodes[MAXN];
int parent[MAXN];
void dfs(int node)
{
pos[node]=true;siz++;
for (int sled:adj[node])
{
if (!pos[sled]) dfs(sled);
}
}
void dfstree(int node,int pret,int depth)
{
dist[node]=depth;nodes[dist[node]].push_back(node);
parent[node]=pret;
for (int sled:adj[node])
{
if (sled!=pret) dfstree(sled,node,depth+1);
}
}
int32_t main()
{
cin>>n;
for (int i=1;i<=n;i++)
{
string s1,s2;cin>>s1>>s2;edges.push_back({s1,s2});
kompresija.push_back(s1);kompresija.push_back(s2);
}
if (n%2==1) {cout<<-1<<endl;return 0;}
sort(kompresija.begin(),kompresija.end());kompresija.erase(unique(kompresija.begin(),kompresija.end()),kompresija.end());
for (pair<string,string> edge:edges)
{
int u=lower_bound(kompresija.begin(),kompresija.end(),edge.first)-kompresija.begin()+1;
int v=lower_bound(kompresija.begin(),kompresija.end(),edge.second)-kompresija.begin()+1;
loved[v]=true;
}
bool subtask2=true;
for (int i=1;i<=(int)kompresija.size();i++)
{
if (!loved[i]) {subtask2=false;break;}
}
if (subtask2)
{
for (pair<string,string> edge:edges)
{
int u=lower_bound(kompresija.begin(),kompresija.end(),edge.first)-kompresija.begin()+1;
int v=lower_bound(kompresija.begin(),kompresija.end(),edge.second)-kompresija.begin()+1;
adj[u].push_back(v);
}
int answer=0;
for (int node=1;node<=(int)kompresija.size();node++)
{
if (pos[node]) continue;
siz=0;dfs(node);if (siz==2) continue;
answer+=(siz+1)/2;
}
cout<<answer<<endl;return 0;
}
for (pair<string,string> edge:edges)
{
int u=lower_bound(kompresija.begin(),kompresija.end(),edge.first)-kompresija.begin()+1;
int v=lower_bound(kompresija.begin(),kompresija.end(),edge.second)-kompresija.begin()+1;
if (u!=v) {adj[u].push_back(v);adj[v].push_back(u);}
}
dfstree(1,0,0);int maxdist=-1;for (int node=1;node<=n;node++) maxdist=max(maxdist,dist[node]);
int answer=0;
for (int d=maxdist;d>=1;d--)
{
for (int node:nodes[d])
{
if (pos[node] or (int)adj[node].size()==1) continue;
answer++;pos[node]=true;pos[parent[node]]=true;
}
}
cout<<n-answer<<endl;
return 0;
}
