#include "bits/stdc++.h"
using namespace std;
#define SZ(s) (int)s.size()
#define ss second
#define ff first
const int N = 1e5+5;
map <string, int> mp;
vector <int> p, cn, vis;
int n, k, ans;
void dfs(int x){
vis[x] = true;
int y = p[x];
if(y == x or vis[y]){
ans++;
return;
}
vis[y] = true;
if(p[y] == x) return;
ans++;
if(!vis[p[y]]) dfs(p[y]);
}
int main(){
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
int cnt = 0;
p.assign(n+1, 0), cn.assign(n+1,0);
for(int i = 0; i < n; i++){
string s, t;
cin >> s >> t;
if(mp.find(s) == mp.end()){
mp[s] = (++cnt);
}
if(mp.find(t) == mp.end()){
mp[t] = (++cnt);
}
p[mp[s]] = mp[t];
cn[mp[t]]++;
}
if(n % 2 == 1) {
cout << -1;
return 0;
}
vis.assign(n+1,0);
set <pair<int,int>> s;
for(int i = 1; i <= n; i++){
int y = p[i];
if(p[y] == i and y != i) vis[i] = vis[y] = true;
}
for(int i = 1; i <= n; i++){
if(vis[i]) continue;
s.insert({cn[i], i});
}
while(SZ(s)){
auto [z, x] = *s.begin();
if(z) break;
int y = p[x];
s.erase(s.begin());
vis[x] = true;
if(vis[y] or y == x){
ans++;
continue;
}
ans++;
s.erase(s.find({cn[y],y}));
vis[y] = true;
if(p[y] == y or vis[p[y]]) continue;
s.erase(s.find({cn[p[y]],p[y]}));
cn[p[y]]--;
s.insert({cn[p[y]],p[y]});
}
for(int i = 1; i <= n; i++){
if(vis[i]) continue;
dfs(i);
}
cout << ans;
return 0;
}
