#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (int i = a; i < b; ++i)
#define per(i, a, b) for (int i = b - 1; i >= a; --i)
#define trav(a, x) for (auto& a : x)
#define sz(a) a.size()
#define umap unordered_map
#define uset unordered_set
typedef pair<int, int> ii;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<vi> vvi;
umap<string, string> out;
umap<string, int> in;
uset<string> unloved;
int ans = 0, n;
void remove(string name) {
if (in.count(out[name]) && 1 == in[out[name]]--) unloved.insert(out[name]);
out.erase(name);
in.erase(name);
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
if (n & 1) {
cout << -1 << endl;
return 0;
}
string n1, n2;
vector<string>arr(n*2);
rep(i, 0, n) {
cin >> arr[i] >> arr[i + n];
out.emplace(arr[i], arr[i + n]);
in.emplace(arr[i],0);
}
cout<<endl;
vector<string> rem;
rep(i, 0, n) {
n1 = arr[i];
n2 = arr[i + n];
if(n1==n2)continue;
if (out[n2] == n1) rem.push_back(n1);
++in[n2];
}
trav(name, rem) {
out.erase(name);
in.erase(name);
}
trav(person,in)if(!person.second)unloved.insert(person.first);
while (!unloved.empty()) {
string name = *unloved.begin();
unloved.erase(unloved.begin());
if (!out.count(name) || !out.count(out[name])) continue;
++ans;
remove(out[name]);
out.erase(name);
in.erase(name);
}
cout << ans+sz(in) << endl;
}
