#ifndef LOCAL
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC diagnostic ignored "-Wpedantic"
#endif
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
auto seed = chrono::high_resolution_clock::now().time_since_epoch().count();
mt19937 rnd(52);
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename V>
using table = gp_hash_table<T, V>;
using i128 = __int128;
using ll = long long;
using ld = long double;
using uint = unsigned int;
using ull = unsigned long long;
const ll INF = 2e18;
const int inf = 1e9;
const int maxn = 1e5 + 7;
const int MOD = 1e9 + 7;
const ld pi = acos(-1);
const int P = 5167;
const int L = 26;
const ld EPS = 1e-7;
template<typename T, typename V>
void fill(T &container, V value) {
for (auto &c : container)
c = value;
}
vector<int> g[maxn];
int dp[maxn];
void dfs(int v) {
for (auto to : g[v]) {
dfs(to);
dp[v] = max(dp[v], 1 - dp[to]);
}
}
void solve() {
int n;
cin >> n;
if (n % 2 == 1) {
cout << -1;
return;
}
map<string, int> mp;
int curr = 1;
vector<int> roots;
for (int i = 0; i < n; ++i) {
string s, t;
cin >> s >> t;
if (mp[s] == 0) mp[s] = curr++;
if (mp[t] == 0) mp[t] = curr++;
if (s == t) roots.push_back(mp[s]);
else g[mp[t]].push_back(mp[s]);
}
for (auto root : roots) {
dfs(root);
}
int ans = n;
for (auto c : dp) {
ans -= c;
}
cout << ans;
}
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout << fixed << setprecision(4);
int t = 1;
//cin >> t;
while (t--) solve();
//stress();
}
