#include <bits/stdc++.h>
using namespace std;
int const N=1e5+5;
map<string,int> mp;
int suc[N];
vector<int> lover[N];
vector<int> adj[N];
vector<int> cc[N];
int c=0;
int ccn[N];
void dfs(int node){
ccn[node]=c;
cc[c].push_back(node);
for(auto i:adj[node])
if(ccn[i]==0)
dfs(i);
}
// int largest_independent_set(int n){
// return //;
// }
int main(){
int n;
cin>>n;
string a,b;
int sz=1;
for (int i = 0; i < n; ++i)
{
cin>>a>>b;
if(mp[a]==0){
mp[a]=sz;
sz++;
}
if(mp[b]==0){
mp[b]=sz;
sz++;
}
suc[mp[a]]=mp[b];
lover[mp[b]].push_back(mp[a]);
adj[mp[b]].push_back(mp[a]);
adj[mp[a]].push_back(mp[b]);
}
if(n%2){
cout<<-1<<endl;
return 0;
}
for (int i = 1; i <=n; ++i)
if(ccn[i]==0){
c++;
dfs(i);
}
int ans=0;
for (int i = 1; i <=c; ++i)
{
int k=cc[i].size();
ans+=k/2;
ans+=k%2;
}
cout<<ans<<endl;
return 0;
}
