//Bismi Allah
#include "bits/stdc++.h"
using namespace std;
signed main () {
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n;
cin >> n;
map <string,int> mp;
vector <int> used(n, 0), dp(n, 0), g[n], unite(n, 0);
function <void(int,int)> dfs=[&](int v, int p) {
used[v] = 1;
for(auto u : g[v]) {
if(used[u]) continue;
dfs(u, v);
}
if(unite[v]) return;
if(p != -1 && unite[p] == 0) {
unite[p] = 1;
unite[v] = 1;
}
};
for(int i = 0; i < n; i ++) {
string u, v;
cin >> u >> v;
if(mp.count(u) == 0) mp[u] = mp.size();
if(mp.count(v) == 0) mp[v] = mp.size();
if(mp[u] == mp[v]) continue;
g[mp[u]].push_back(mp[v]);
g[mp[v]].push_back(mp[u]);
}
if(n % 2 == 1) {
// assert(false);
cout << -1;
return 0;
}
int ans = 0, mod = 0;
for(int i = 0; i < n; i ++) {
if(used[i]) continue;
dfs(i, -1);
}
for(auto i : unite) {
ans += (i == 1);
mod += (i == 0);
}
cout << ans/2+mod;
return 0;
}
