#include <bits/stdc++.h>
using namespace std;
typedef vector<int> vi;
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++;
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);
set<int> in_cycles;
for (int i = 0; i < n; i++)
in_cycles.insert(i);
while (q.size()) {
int u = q.front(); q.pop();
int v = graph[u];
in_cycles.erase(u);
if (v == -1) {
res++;
continue;
}
else if (graph[v] != -1 && graph[graph[v]] == v) {
res++;
continue;
}
in_cycles.erase(v);
for (auto x : rgraph[v])
graph[x] = -1;
int w = graph[v];
if (w != -1) {
rgraph[w].erase(v);
if (rgraph[w].size() == 0)
q.push(w);
}
res++;
}
vector<bool> visited(n, false);
for (auto u : in_cycles) {
if (visited[u]) continue;
int v = u, len = 0;
while (!visited[v]) {
visited[v] = true;
v = graph[v];
len++;
}
if (len != 2)
res += (len + 1) / 2;
}
cout << res << '\n';
return 0;
}
