// Source: https://usaco.guide/general/io
#include <bits/stdc++.h>
using namespace std;
typedef vector<int> vi;
typedef vector<vi> vvi;
int num_pairs=0;
int num_paired=0;
vector<int> last_vis;
vector<bool> is_polygon;
vector<bool> taken;
vvi G;
void dfs_pair(int node, int par){
for(int neighbor : G[node]){
if(neighbor==par) continue;
dfs_pair(neighbor, node);
}
if(par==node) return;
if(!taken[par] && !taken[node]){
num_paired++;
taken[par]=1;
taken[node]=1;
}
}
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
int n; cin>>n;
if(n%2==1){
cout<<-1;
return 0;
}
//need to get the input
vector<string> names(n);
map<string, int> func;
for(int i=0; i<n; i++){
string s;
cin>>s;
func[s]=i;
cin>>s;
names[i]=s;
}
vi loves(n);
for(int i=0; i<n; i++){
loves[i]=func[names[i]];
}
//found the thing
last_vis.assign(n, -1);
is_polygon.assign(n, 0);
taken.assign(n, 0);
for(int i=0; i<n; i++){
last_vis[i]=i;
int curr=loves[i];
while(last_vis[curr]==-1){
last_vis[curr]=i;
curr=loves[curr];
}
if(last_vis[curr]==i){
//we have a polygon starting with curr
int l=0;
while(is_polygon[curr]==0){
is_polygon[curr]=1;
curr=loves[curr];
l++;
}
if(l==2){
num_pairs++;
taken[curr]=1;
taken[loves[curr]]=1;
}
}
else continue;
}
G.resize(n);
for(int i=0; i<n; i++){
if(is_polygon[i]) continue;
G[i].push_back(loves[i]);
G[loves[i]].push_back(i);
//graph
}
//found all polygons. now time to root from all of them and do dp
for(int i=0; i<n; i++){
if(is_polygon[i]){
dfs_pair(i, i);
}
}
vector<bool> vis(n, 0);
for(int i=0; i<n; i++){
if(!is_polygon[i] || vis[i]) continue;
int curr=i;
int l=0;
while(true){
if(taken[curr]){
//we know what to do here
//start the loop from here and count the lengths of the free segments
int start=curr;
l=0;
curr=loves[curr];
vis[start]=1;
vis[curr]=1;
while(curr!=start){
if(taken[curr]){
num_paired+=l/2;
l=0;
}
else l++;
vis[curr]=1;
curr=loves[curr];
}
num_paired+=l/2;
break;
}
else{
if(vis[curr]){
num_paired+=l/2;
break;
}
l++;
vis[curr]=1;
curr=loves[curr];
}
}
}
//just need to find a way to pair the polygons themselves
cout<<n-num_paired-2*num_pairs;
return 0;
}
