#include <bits/stdc++.h>
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
using namespace std;
vector<int> a;
vector<int> req;
int n;
vector<int> neigh;
vector<int> visited;
int rst=0;
void dfs(int x){
if(neigh[x]==x||visited[neigh[x]]) {
rst++;
return;
}
visited[x]=true;
visited[neigh[x]]=true;
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
neigh.resize(n);
visited.resize(n);
map<string,int> mp;
int t=0;
for (int i = 0; i < n; i++)
{
string sa; string b; cin >> sa >> b;
if(mp.find(sa)==mp.end()) mp[sa]=t++;
if(mp.find(b)==mp.end()) mp[b]=t++;
neigh[mp[sa]]=mp[b];
}
for (int i = 0; i < n; i++)
{
if(!visited[i]){
dfs(i);
}
}
if(n%2) cout << -1 << "\n";
else cout << n/2+rst/2;
return 0;
}
