#include <bits/stdc++.h>
using namespace std;
typedef vector<int> vi;
int cycle_length(int i, int start, vi& graph, vector<bool>& visited, int length) {
visited[i] = true;
if (i == start) return length;
else return cycle_length(graph[i], start, graph, visited, length+1);
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
map<string,int> names;
int cur_name = 0;
vi graph(n);
vector<set<int>> rgraph(n);
for (int i = 0; i < n; i++) {
string a, b;
cin >> a >> b;
if (names.find(a) == names.end()) names[a] = cur_name++;
if (names.find(b) == names.end()) names[b] = cur_name++;
if (names[a] == names[b])
graph[names[a]] = -1;
else {
graph[names[a]] = names[b];
rgraph[names[b]].insert(names[a]);
}
}
if (n & 1) {
cout << -1 << '\n';
return 0;
}
int res = 0;
queue<int> q;
for (int i = 0; i < n; i++)
if (rgraph[i].size() == 0)
q.push(i);
while (q.size()) {
int u = q.front(); q.pop();
int v = graph[u];
if (v == -1)
continue;
res++;
for (auto x : rgraph[v])
if (x != u)
graph[x] = -1;
int w = graph[v];
if (w > -1) {
rgraph[w].erase(rgraph[w].find(v));
if (rgraph[w].size() == 0)
q.push(w);
}
graph[v] = u;
}
vector<bool> visited(n, false);
for (int i = 0; i < n; i++) {
if (visited[i]) continue;
visited[i] = true;
if (graph[i] == -1) res++;
else {
int c = cycle_length(graph[i], i, graph, visited, 1);
if (c > 2)
res += (c + 1) / 2;
}
}
cout << res << '\n';
return 0;
}
