#include <bits/stdc++.h>
using namespace std;
vector<int> points_to;
vector<set<int>> pointed_by;
vector<bool> solved;
vector<bool> useless;
int ans = 0;
int useless_count = 0;
void resolve_leaf(int u) {
if (0 < pointed_by[u].size() || solved[u])
return;
if (solved[points_to[u]] && !useless[points_to[u]]) {
useless_count++;
solved[u] = 1;
return;
}
int par = points_to[u];
int gpar = points_to[par];
pointed_by[gpar].erase(par);
for (auto v : pointed_by[par])
if (v != u && 0 == pointed_by[v].size()) {
useless_count++;
solved[v] = 1;
useless[v] = 1;
}
points_to[points_to[u]] = u;
pointed_by[u].insert(par);
solved[u] = 1;
solved[points_to[u]] = 1;
ans++;
if (useless[points_to[u]]) {
useless_count--;
useless[points_to[u]] = 0;
}
resolve_leaf(gpar);
}
void resolve_cycle(int u, int sx = -1) {
if (solved[u])
return;
if (u == sx)
return;
if (sx == -1)
sx = u;
int par = points_to[u];
int gpar = points_to[par];
if (par == sx) {
ans += 1;
solved[u] = 1;
return;
}
solved[u] = 1;
solved[par] = 1;
ans++;
if (gpar != sx)
resolve_cycle(gpar);
}
int main() {
int n;
cin >> n;
points_to.resize(n, -1);
pointed_by.resize(n);
unordered_map<string, int> name_to_idx;
solved.resize(n);
useless.resize(n);
for (int i = 0; i < n; i++) {
string a, b;
cin >> a >> b;
if (!name_to_idx.count(a))
name_to_idx[a] = name_to_idx.size();
if (!name_to_idx.count(b))
name_to_idx[b] = name_to_idx.size();
int aidx = name_to_idx[a], bidx = name_to_idx[b];
// cerr << "[" << a << "][" << b << "]" << endl;
// cerr << aidx << " " << bidx << endl;
points_to[aidx] = bidx;
pointed_by[bidx].insert(aidx);
if (aidx == bidx) {
solved[aidx] = 1;
useless[aidx] = 1;
useless_count++;
} else if (points_to[points_to[aidx]] == aidx) {
solved[aidx] = 1;
solved[bidx] = 1;
}
}
if (n % 2 == 1) {
cout << -1 << endl;
return 0;
}
for (int a = 0; a < n; a++)
resolve_leaf(a);
for (int a = 0; a < n; a++)
if (points_to[a] != a && !solved[a])
resolve_cycle(a);
ans += useless_count;
cout << ans << endl;
return 0;
}
