#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll SIZE = 100100;
bool info[SIZE];
ll cnt_matching(ll node, ll p, ll cnt, bool ignore_cyc, vector<ll> &vis, vector<vector<ll>> &g, vector<bool> &matched){
vis[node] = cnt;
ll ans = 0;
for(ll i: g[node]) if((!vis[i] || (ignore_cyc && vis[i] == 1)) && (ignore_cyc || !info[i]) && !matched[i]){
ans += cnt_matching(i, node, cnt, ignore_cyc, vis, g, matched);
if(!matched[i] && !matched[node]) matched[i] = matched[node] = 1, ans++;
}
return ans;
}
ll cnt_for_cyc(vector<ll> &cyc, vector<bool> &matched, ll &pairs){
ll s = 0, n = cyc.size();
for(ll i = 0; i < n; i++) if(matched[cyc[i]]) s = i;
if(!matched[cyc[s]]) {
if(n == 2) {pairs++; return 0;}
return n/2;
}
ll cnt = 0, ans = 0;
for(ll i = (s+1) % n; i != s; i = (i+1) % n){
if(matched[cyc[i]]) ans += cnt/2, cnt = 0;
else cnt++;
}
ans += cnt/2;
return ans;
}
ll solve(ll n, vector<ll> &edge, vector<vector<ll>> &rg){
if(n % 2) return -1;
vector<vector<ll>> cycles;
vector<ll> vis(n+1);
ll cnt = 0;
for(ll i = 1; i <= n; i++) if(!vis[i]){
ll node = i; cnt++;
vector<ll> v;
while(!vis[node]) v.push_back(node), vis[node] = cnt, node = edge[node];
if(vis[node] == cnt) {
ll x = node;
vector<ll> cyc;
while(v.size() && v.back() != x) info[v.back()] = 1, cyc.push_back(v.back()), v.pop_back();
info[node] = 1, cyc.push_back(node);
cycles.push_back(cyc);
}
}
vector<vector<ll>> rg_same_dir_cyc(n+1);
for(ll i = 1; i <= n; i++) {
if(info[i] && info[edge[i]]) rg_same_dir_cyc[i].push_back(edge[i]);
else rg_same_dir_cyc[edge[i]].push_back(i);
}
vector<ll> cnt_matching_vis(n+1);
vector<bool> matched(n+1);
ll ans = 0, pairs = 0, at = 0;
for(vector<ll> cyc: cycles){
ll node = 0;
for(ll idx = 0; idx < cyc.size(); idx++){
ll i = cyc[idx];
ans += cnt_matching(i, 0, 1, 0, cnt_matching_vis, rg, matched);
if(matched[i]){
at = idx; node = i; break;
}
}
if(node){
ans += cnt_matching(cyc[(at + 1) % cyc.size()], 0, cnt, 1, cnt_matching_vis, rg, matched);
}
else if(cyc.size() == 2) pairs++;
else ans += cyc.size() / 2;
}
return ans + n - 2*(pairs + ans);
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(NULL);
ll n, cnt = 0; cin >> n;
vector<ll> edge(n+1);
vector<vector<ll>> rg(n+1);
map<string, ll> names;
for(ll i = 0; i < n; i++){
string s, t; cin >> s >> t;
if(!names[s]) names[s] = ++cnt;
if(!names[t]) names[t] = ++cnt;
edge[names[s]] = names[t]; rg[names[t]].push_back(names[s]);
}
cout << solve(n, edge, rg);
}
