#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
map<string, int> mp;
vector<int> to(n);
for (int i = 0; i < n; i++) {
string s;
cin >> s;
string t;
cin >> t;
if (!mp.count(s)) {
mp[s] = (int) mp.size();
}
if (!mp.count(t)) {
mp[t] = (int) mp.size();
}
to[mp[s]] = mp[t];
}
if (n % 2 == 1) {
cout << -1 << '\n';
return 0;
}
vector<vector<int>> ch(n);
for (int i = 0; i < n; i++) {
ch[to[i]].push_back(i);
}
vector<int> deg(n);
for (int i = 0; i < n; i++) {
deg[i] = (int) ch[i].size();
}
vector<int> que;
for (int i = 0; i < n; i++) {
if (deg[i] == 0) {
que.push_back(i);
}
}
for (int b = 0; b < (int) que.size(); b++) {
int i = que[b];
deg[to[i]]--;
if (deg[to[i]] == 0) {
que.push_back(to[i]);
}
}
for (int i = 0; i < n; i++) {
if (deg[i] > 0) {
que.push_back(i);
}
}
vector<vector<int>> dp(n, vector<int>(2));
for (int b = 0; b < (int) que.size(); b++) {
int i = que[b];
for (int j : ch[i]) {
if (i == j) {
continue;
}
dp[i][0] += dp[j][0];
dp[i][1] = max({dp[i][0] + dp[j][0] + 1, dp[i][1] + dp[j][1], dp[i][1] + dp[j][0], dp[i][0] + dp[j][1]});
}
}
int ans = n;
for (int i = 0; i < n; i++) {
if (to[i] == i) {
ans -= max(dp[i][0], dp[i][1]);
}
}
cout << ans << '\n';
return 0;
}
