#include <bits/stdc++.h>
using namespace std;
#define int long long
const int INF = 1e18;
const int N = 200005;
int n, love[N], in_degree[N];
bool matched[N];
map<string, int> ids;
int c = 0;
int get_id(string &s) {
int &id = ids[s];
if (id == 0) id = ++c;
return id;
}
signed main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
if (n % 2 != 0) {
cout << "-1\n";
return 0;
}
for (int i = 1; i <= n; i++) {
string u_s, v_s;
cin >> u_s >> v_s;
int u = get_id(u_s), v = get_id(v_s);
love[u] = v;
}
int couples = 0;
for (int i = 1; i <= n; i++) {
if (!matched[i]) {
int j = love[i];
if (i != j && love[j] == i) {
matched[i] = true;
matched[j] = true;
couples += 2;
}
}
}
for (int i = 1; i <= n; i++) {
if (!matched[i]) in_degree[love[i]]++;
}
queue<int> q;
for (int i = 1; i <= n; i++) {
if (in_degree[i] == 0 and !matched[i]) q.push(i);
}
while (!q.empty()) {
int u = q.front();
q.pop();
if (matched[u]) continue;
int v = love[u];
if (!matched[v]) {
matched[u] = true;
matched[v] = true;
couples++;
int T1 = love[v];
if (matched[T1]) continue ;
in_degree[T1]--;
if (in_degree[T1] == 0 ) q.push(T1);
}
}
for (int i = 1; i <= n; i++) {
if (!matched[i]) {
int curr = i, siz = 0;
while (!matched[curr]) {
matched[curr] = true;
curr = love[curr];
siz++;
}
couples += (siz / 2);
}
}
int res = n - couples;
cout << res << '\n';
}
