#include<unordered_map>
#include<unordered_set>
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<iomanip>
#include<cstring>
#include<cassert>
#include<numeric>
#include<vector>
#include<string>
#include<chrono>
#include<random>
#include<stack>
#include<queue>
#include<cmath>
#include<set>
#include<map>
#include<ios>
using namespace std;
signed main() {
int x = 0, n; cin >> n;
vector<unordered_set<int>>gp(n);
unordered_map<string, int>mp;
for (int i = 0; i < n; i++) {
string a, b; cin >> a >> b;
if (mp.find(a) == mp.end()) mp[a] = x++;
if (mp.find(b) == mp.end()) mp[b] = x++;
int u = mp[a], v = mp[b];
gp[u].insert(v);
}
if (n % 2) {
cout << -1;
return 0;
}
if (n <= 20) {
vector<pair<int, int>>dp((1 << n), { 1e9,1e9 });
dp[0] = { 0,0 };
for (int i = 0; i < (1 << n); i++) {
int cnt = 0;
for (int j = 0; j < n; j++) if ((1 << j) & i) cnt++;
if (cnt % 2 == 0) {
for (int j = 0; j < n; j++) {
if ((1 << j) & i) continue;
if (dp[i + (1 << j)].first > dp[i].first) {
dp[i + (1 << j)].first = dp[i].first;
dp[i + (1 << j)].second = j;
}
}
}
else {
int x = dp[i].second;
for (int j = 0; j < n; j++) {
if ((1 << j) & i) continue;
int cnt = 0;
if (gp[x].find(j) == gp[x].end()) cnt++;
if (gp[j].find(x) == gp[j].end()) cnt++;
dp[i + (1 << j)].first = min(dp[i + (1 << j)].first, dp[i].first + cnt);
}
}
}
cout << dp[(1 << n) - 1].first;
return 0;
}
bool fl = 0;
int cnt = 0, ax = 0, ans = 0, ans1 = 0;
queue<int>q;
vector<int>vis(n);
//cout << "----\n";
for (int i = 0; i < n; i++) {
if (vis[i]) continue;
cnt = 0;
bool fl1 = 0;
vis[i] = i + 1;
q.push(i);
while (!q.empty()) {
int u = q.front();
q.pop();
cnt++;
for (auto& v : gp[u]) {
if (vis[v]) {
//cout << u << " " << v << endl;
if (vis[u] == vis[v] && u != v) fl1 = 1;
continue;
}
q.push(v);
vis[v] = i + 1;
}
}
fl |= fl1;
if (cnt != 2) {
if (cnt % 2)ax++;
ans += cnt / 2;
}
}
//cout << "----\n";
//cout << endl << "fl = " << fl << endl;
if (!fl) {
//cout << "brat\n";
cnt = 0, ans = 0;
vector<int>indeg(n),par(n);
stack<int>st;
vis = vector<int>(n);
for (int i = 0; i < n; i++) {
for (auto& j : gp[i]) indeg[j]++, par[i] = j;
}
for (int i = 0; i < n; i++) if (indeg[i] == 0) {
st.push(i);
}
for (int i = 0; i < n; i++) if (par[i] == i) indeg[i]--;
while (!st.empty()) {
int u = st.top();
st.pop();
int v = par[u];
if (indeg[v] == 1) {
vis[u] = vis[v] = true;
indeg[v]--;
ans++;
int x = par[v];
if (x != v) indeg[x]--;
if (indeg[x] == 0) st.push(x);
}
}
for (int i = 0; i < n; i++) if (par[i] == i) indeg[i]++;
for (int i = 0; i < n; i++) if (indeg[i] == 0 && !vis[i]) {
st.push(i);
}
for (int i = 0; i < n; i++) if (par[i] == i) indeg[i]--;
while (!st.empty()) {
int u = st.top();
st.pop();
int v = par[u];
indeg[v]--;
if (indeg[v] == 0) {
vis[v] = vis[u] = true;
int x = par[v];
ans++;
if (x != v) indeg[x]--;
if (x != v && indeg[x] == 0) st.push(x);
}
else {
cnt++;
vis[u] = true;
}
}
for (int i = 0; i < n; i++) ans += vis[i] == 0;
cout << ans + cnt << endl;
}
else cout << ans + ax << endl;
}
